fix: serialization null tag's value into LineProtocol

[bednar]

Closes #282

Proposed Changes

Fixed serialization a Point into LineProtocol when tag's value is null.

Checklist

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

[codecov-commenter]

Codecov Report

Merging #283 (05e18b2) into master (f2ef610) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #283   +/-   ##
=========================================
  Coverage     89.71%   89.71%           
  Complexity      473      473           
=========================================
  Files           149      149           
  Lines          5912     5912           
  Branches        283      283           
=========================================
  Hits           5304     5304           
  Misses          524      524           
  Partials         84       84           

Impacted Files	Coverage Δ
...src/main/java/com/influxdb/client/write/Point.java	90.72% <100.00%> (ø)

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