This traceback occurs when the user makes the `end date` as empty while 
creating a new planning slot with template.

To reproduce this issue:

1) Install planning
2) Create a new `planning slot`
3) click on any `template id`
4) Now remove the `end date`
5) A traceback occurs

Error:-
```
TypeError: unsupported operand type(s) for -: 'bool' and 'datetime.datetime'
```

When the user removes the `end datetime` its value is `False` 
which leads to the above traceback when `-` is used between `start` and `end` datetime.

https://github.com/odoo/enterprise/blob/5600b5b754f79e5d7a253fe9db45ac8a9273d3de/planning/models/planning.py#L406-L408

Additionally, a `value error` is also occurring when the `start` date-time is empty,
while calculating start-end-dates with `template` .  As it returns both `datetime` and timezone.

From the below lines `tzinfo` only removes when the start date time and template is present. 

https://github.com/odoo/enterprise/blob/5600b5b754f79e5d7a253fe9db45ac8a9273d3de/planning/models/planning.py#L621-L627

After applying this commit will resolve both issues and make the code more robust

sentry-5005245970

Forward-Port-Of: odoo/enterprise#61024

The SUN number is normally mandatory to generate such a file, and is repeated at multiple places within it. However, HSBC bank diverges from the others, and allows putting 'HSBC' at a normally unused position of the file; in such case, all the fields supposed to contain the SUN can be left blank.

UHL1 records of the file also use the SUN to build an identifier. HSBC's doc is unclear about what should go there, but ING says here https://www.ingwb.com/binaries/content/assets/support-content/payments-and-reporting/insidebusiness-payments/domestic-uk-file-format-description-february-2024.pdf that this identifier should be 999999 + 4 blanks or SUN + 4 blanks, so we make the choice to use 999999 if we have no SUN, for HSBC.

We don't want to make the SUN field optional for everyone just because of that one use case. Instead, we ask people to write 'HSBC' as their SUN number in the settings if they don't have any, and this considers the field as blank for the file generation.

(HSBC's doc can be found on the related ticket)
opw-3863262

Forward-Port-Of: odoo/enterprise#62224
Forward-Port-Of: odoo/enterprise#62111