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

[fergusean]

Summary of changes

Same as #552, but for dates. Can't use Date | null on SQL entities right now without Deepkit trying to cast them as JSON into the database.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.58%. Comparing base (51b9b36) to head (d146d79).

❗ Current head d146d79 differs from pull request most recent head c5760c1. Consider uploading reports for the commit c5760c1 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #553   +/-   ##
=======================================
  Coverage   78.58%   78.58%           
=======================================
  Files         193      193           
  Lines       20448    20448           
  Branches     5311     5311           
=======================================
  Hits        16070    16070           
  Misses       4378     4378           

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[marcj]

thanks @fergusean! I also adjusted sqlite/postgres to use the new date type resolver ab12802