Currently, the behavior of the "Maximum closing difference" feature with employees depends on the user connected in the backend and not the employee using the pos.

Steps to reproduce:
-------------------
* Set max closing difference as 0
* Have 1 admin user and 1 pos user
* Have 2 employees
* Set admin user and employee 1 as advanced employees of the pos
* Set pos user and employee 2 as basic employees

Steps with admin:
* Make sure you are logged as the admin in the database
* Open pos (could be a session opened by other user)
* Log in with Admin user
* Try to close the pos with a difference of 10 -> You can, ok
* Log in with employee 1 (advanced)
* Try to close the pos with a difference of 10 -> You ca but shouldn't

Steps with pos user
* Now log in the database as pos user
* Open pos (could be a session opened by other user
* Log in with employee 1 (advanced)
* Try to close the pos with a difference of 10 -> You cannot, ok
* Log in with Admin user
* Try to close the pos with a difference of 10 -> you cannot but should

Why the fix:
------------
Employees that have no linked user should not ba able to override the max difference.

Employees who have a connected user should only be able to override the max difference if their user is admin of the pos.

opw-5184041

Forward-Port-Of: odoo/odoo#240978
Forward-Port-Of: odoo/odoo#235356

Currently, an error occurs when a user attempts to pay an invoice.

**Steps to Reproduce ([Video](https://drive.google.com/file/d/1onmo1mxeZgH6fkQOoueGCgC67HPjYHX6/view)):**

 - Install the `Accounting` module.
 - Go to `Payment Terms` and create `a new Payment Term` with at `least two Due Term lines`.
 - Go to `Invoices` and `create a new Invoice`.
 - Add one invoice line and set the `Payment Term` to the `newly created payment term`.
 - In the `Journal Items` tab > `Enable the Due Date` column (optional hidden).
 - From the two `Receivable journal items`, remove the `Due Date` from one of the `receivable lines`.
 - Now `Confirm the invoice` and `Click on Pay`.

**Error:**
`TypeError: '<' not supported between instances of 'datetime.date' and 'bool'`

**Cause:**
This error occurs when the user clicks Pay, than it going to calculate the total amount to pay from here [1]. If the payment term has more than one term line, it creates more than one receivable invoice line, and the receivable invoice lines are sorted from here [2]. When two or more invoice lines have the same move_id, they are sorted based on the due date. However, if one of the receivable lines does not have a due date, the error is raised.
Similarly, as shown in [3], when the system retrieves the installment data, it sorts the lines based on the due date and raises the same error.

**Fix:**
This commit ensures that when there is no due date on any receivable invoice line and two lines belong to the same invoice, the comparison uses the maximum date as like here [4] and places that line at the end for that invoice, thereby maintaining the correct flow. The same fix is applied while retrieving the installment data, as described above.

[1]: https://github.com/odoo/odoo/blob/92a9f6b19670685dfe9fb1714bf01449768e5f62/addons/account/wizard/account_payment_register.py#L703

[2]- https://github.com/odoo/odoo/blob/92a9f6b19670685dfe9fb1714bf01449768e5f62/addons/account/wizard/account_payment_register.py#L635

[3]: https://github.com/odoo/odoo/blob/92a9f6b19670685dfe9fb1714bf01449768e5f62/addons/account/models/account_move_line.py#L3305

[4]: https://github.com/odoo/odoo/blob/92a9f6b19670685dfe9fb1714bf01449768e5f62/addons/account/models/account_move_line.py#L522

sentry-713241246

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Forward-Port-Of: odoo/odoo#241744
Forward-Port-Of: odoo/odoo#241060

## Issue:
When exporting the XML for a VAT return, the client nihil field was incorrectly set to YES
According to Belgian regulations, this is only allowed on the last report of the calendar year
https://finances.belgium.be/sites/default/files/downloads/165-625-directives-2019.pdf

## Cause:
Prior to 18.3, nihil was a checkbox option on export
Since 18.3, it is automatically selected based solely on a price formula, without checking the report’s date

This PR brings back the Client Nihil checkbox in 19.0
But didn't change the default value
Causing the issue to still be present
https://github.com/odoo/enterprise/pull/99496

## Steps to reproduce:
- Select the Belgian company
- Open the Tax Return and select the VAT Return
- Click Returns and enter an opening date in January
- Click Review for the first available report
- Ignore errors and return to previous page
- Click on Submit -> Download XML
- Before the fix, ClientListingNihil is set to YES

## Potential remaining issue:
The legal document specify that it should also be checked in case of "cessation d’activité"
So it may be interesting to consider adding the option back

opw-5184056

Forward-Port-Of: odoo/enterprise#102482
Forward-Port-Of: odoo/enterprise#98982

Before, if you paid for something on the website with ecommerce, it would create a sale order with a transaction linked to it, but no invoice. So in the POS the order would show up in the Quotation/Order menu, but the POS didn't have any reference on the payment, so you would have to create another duplicate payment.

Now the amount_paid (which is the amount already paid in transactions) is taken into account when calculating what's left to pay in the POS. So anytime you open a sale order in the POS it should create a down payment line for anything already paid on the website (the same as we were already doing for invoices).

Also to avoid double payments, now the sale orders which have been already paid in full, even if they are not invoiced yet, will not show up in the POS. The invoice can still be created normally from the Sale app in Odoo.

Task-5138081

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Forward-Port-Of: odoo/odoo#230809