fix: nanosecond datetime string convertor

[bednar]

Closes #94

Proposed Changes

Some APIs can return date without nanosecond precision.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• make test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #96 (31102d7) into master (8f52ccc) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #96      +/-   ##
============================================
+ Coverage     81.66%   81.88%   +0.22%     
  Complexity      379      379              
============================================
  Files            23       23              
  Lines           900      900              
============================================
+ Hits            735      737       +2     
+ Misses          165      163       -2     

Impacted Files	Coverage Δ
src/InfluxDB2/ObjectSerializer.php	56.92% <100.00%> (+1.53%)	⬆️

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