[Backend] Consistent null values

[ds-crehm]

As team,
I want empty values to be saved consistently as null in the backend,
so that they are easy to work with and there are no misunderstandings.

Hints / Details

Backend implementation of #578

Acceptance Criteria

Whenever a string value is saved or updated in the database and during data consumption from IRS:

• Leading and trailing whitespace characters are trimmed.
• Empty strings ("") are all converted to null.
• Consistent persistence of null values in database
• All api GET endpoints
• FE filtering for null values from frontend "-" is supported.

Out of Scope

• Policy Api