Motivation

This PR updates base-repository typing to work correctly in strictNullChecks mode.

Fixes #443

Type of change:

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes