fix(sql): strings should be nullable without casting as JSON

[fergusean]

Summary of changes

The other typeResolvesToX functions filter out undefined/null before determining if the type correctly resolves, but string does not. As a result, string | null types on database entities are being coerced to JSON.

Relinquishment of Rights

Please mark following checkbox to confirm that you relinquish all rights of your changes:

• [X] I waive and relinquish all rights regarding this changes (including code, text, and images) to Deepkit UG (limited), Germany. This changes (including code, text, and images) are under MIT license without name attribution, copyright notice, and permission notice requirement.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3e09ddf) 78.06% compared to head (287c853) 78.06%.

:exclamation: Current head 287c853 differs from pull request most recent head 284bef3. Consider uploading reports for the commit 284bef3 to get more accurate results

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #552 +/- ## ======================================= Coverage 78.06% 78.06% ======================================= Files 191 191 Lines 20296 20296 Branches 5255 5255 ======================================= Hits 15844 15844 Misses 4452 4452 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[marcj]

thanks @fergusean !