Allow null values for optional columns in geopackage

[rkettelerij]

Description

Make extend columns optional according to gpkg spec.

Type of change

• Improvement of existing feature
• Bugfix

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR