fix: correct serialization empty columns into LineProtocol [DataFrame]

[bednar]

Related to https://community.influxdata.com/t/writing-entries-gives-invalid-field-format-error-but-only-if-2-tags-are-added-together-separately-works-fine/22368

Proposed Changes

Fixed regexp for removing leading comma for NaN values.

Checklist

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

[codecov-commenter]

Codecov Report

Merging #359 (1fbd754) into master (81e7d21) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #359   +/-   ##
=======================================
  Coverage   92.81%   92.81%           
=======================================
  Files          27       27           
  Lines        2324     2325    +1     
=======================================
+ Hits         2157     2158    +1     
  Misses        167      167           

Impacted Files	Coverage Δ
...fluxdb_client/client/write/dataframe_serializer.py	98.03% <100.00%> (+0.01%)	⬆️

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