[backend] nil filter operators for empty strings and dates (#6517)

[Archidoit]

• backend: nil and not_nil operators now handles empty string and empty dates
• frontend: nil and not_nil operators now available for dates filters (exemple: last_seen for malwares, publication date for reports, etc)

issue: https://github.com/OpenCTI-Platform/opencti/issues/6517

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.34711% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.76%. Comparing base (34ca2e8) to head (17d131a). Report is 2 commits behind head on master.

Files	Patch %	Lines
...ti-platform/opencti-graphql/src/database/engine.js	98.34%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6550      +/-   ##
==========================================
+ Coverage   67.70%   67.76%   +0.06%     
==========================================
  Files         570      570              
  Lines       70592    70676      +84     
  Branches     6013     6020       +7     
==========================================
+ Hits        47791    47891     +100     
+ Misses      22801    22785      -16     

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[labo-flg]

tested locally ✅