Improve default value handling for NULL value fields
Currently the default value handling for input fields that can be set to NULL (eval is set to null) is quite inconsistent when creating new records:
- if mode is set to useOrOverridePlaceholder the field is NULL by default
- if this setting is not used the default value is a non empty string
Additionally it is not possible to explicitly set a NULL default value.
It would be better to if the mode does not have any influence on the default value but if it is possible to set NULL as default.
When this is handled together with #53662 backward compatibility should not be a problem because currently new records will have NULL values by default but empty strings are stored in the database.