Before this commit, the invoice cancelled with payment state set to "Invoicing App Legacy" is actually not really cancelled, it seems to just be a way to create accounting entry. However, the timesheets are considered as not billed if there is no invoice linked to them or if the invoice is cancelled (that is, if the state is set to "Cancelled"), which means, if the invoice is in "legacy status", the timesheets could move in a new invoice since those ones are considered as not billed.

This commit adds a condition to determine to know if a timesheet is not billed. The timesheet should now be:
- not linked to an invoice
- or the invoice linked should be cancelled and payment status should not be "Invoicing App Legacy".

Steps to reproduce the issue:
----------------------------
1. install Accounting, Sales and Timesheets app
2. Create a SO with a service product and confirm the SO
3. Create a timesheet for the SOL contained the service product
4. Create a invoice to partially invoice the service made for the SO.
5. Go to "Accounting app > Settings"
6. Enable the "Invoicing Switch Threshold" feature and make sure the date set on that feature is most recent than the one set on the invoice created in step 4. By doing that, the invoice created in step 4 should now be cancelled but with payment status set to "Invoicing App Legacy".
7. Create another timesheet for the same SOL
8. Go back to the SO created in step 2 and create another invoice to invoice the another timesheet.

Current behavior:
----------------

The timesheet linked to the first invoice is now linked to the second one with the other timesheet.

Expected behavior:
------------------

Since the first invoice is not really cancelled, the timesheet should still be linked to the invoice and so that timesheet should be considered as billed.

opw-4556956

Forward-Port-Of: odoo/odoo#198669
Forward-Port-Of: odoo/odoo#198277

**Steps to reproduce:**
- Install Accounting
- On Accounting dashboard, click on "Import File" button of Bank journal
- Select a CSV file with bank statements
- On the left menu, select "No Separator" as "Thousands Separator"

**Issue:**
"Comma" is displayed as selected instead the first time.
The second time "No Separator" stays as selected.
However, the value sent when testing or importing is not the correct one.

**Cause:**
"No Separator" option should have an empty string as value.
However, during the generation of the "select" element, the value for the "option" element is evaluated with:
`opt.value or opt`
As `opt.value` is the empty string (evaluated to False), `opt` is used instead, even if it is an object, which is not correct.
The main issue is that the empty string is not handled as a valid value.

**Solution:**
Handle the empty string as an acceptable value.

opw-4325310




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Forward-Port-Of: odoo/odoo#192536