Fix >= and <= operators for date queries

Summary

This MR add two missing branches in the getDateWhereClause function and fixes an consistency in the constructDateTimeWhereClauseForColumn function.

Focus

1. Don't let the whitespace changes confuse you.
2. The changes in constructDateTimeWhereClauseForColumn merely add an "OR value_ns IS NULL".
3. The changes in getDataWhereClause add an ELSEIF branch for each operator.

Test Environment

Nothing special. Have a look at the tests in https://gitlab.com/caosdb/caosdb-pyinttest/-/merge_requests/47

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• All automated tests pass
• Reference related Issues
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  • Intent of new code
  • Problems with old code
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• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
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