fixing empty tracestate header handling

[brettmc]

if an empty tracestate header was passed, we were emitting a warning (discovered in the demo app after enabling logging). to fix this, check for empty string as well as null.

[codecov]

Codecov Report

Merging #936 (6a913bb) into main (88ea7c3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 6a913bb differs from pull request most recent head 17c2776. Consider uploading reports for the commit 17c2776 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #936   +/-   ##
=========================================
  Coverage     82.45%   82.46%           
- Complexity     1938     1939    +1     
=========================================
  Files           253      253           
  Lines          5518     5519    +1     
=========================================
+ Hits           4550     4551    +1     
  Misses          968      968           

Flag	Coverage Δ
7.4	81.68% <100.00%> (+<0.01%)	⬆️
8.0	82.44% <100.00%> (+<0.01%)	⬆️
8.1	82.58% <100.00%> (+<0.01%)	⬆️
8.2	82.58% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/API/Trace/TraceState.php	100.00% <100.00%> (ø)

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