fix: date sorting doesn't work if one of the value is null

[risalfajar]

If one of the columns contain a null value (as in the test cases), the built-in date sorting won't work. I've fixed it.

[coveralls]

Pull Request Test Coverage Report for Build 6231219779

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 74.962%

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Totals
Change from base Build 5824891354:	0.1%
Covered Lines:	810
Relevant Lines:	1031

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💛 - Coveralls