fix implicit null usage

[brettmc]

php8.4 deprecates implicit null (now checked by phan)

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.86%. Comparing base (80e97da) to head (255f267).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/API/Baggage/BaggageBuilder.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1362      +/-   ##
============================================
- Coverage     73.94%   73.86%   -0.09%     
  Complexity     2641     2641              
============================================
  Files           379      379              
  Lines          7546     7545       -1     
============================================
- Hits           5580     5573       -7     
- Misses         1966     1972       +6     

Flag	Coverage Δ
8.1	73.59% <91.66%> (-0.02%)	⬇️
8.2	73.77% <91.66%> (+<0.01%)	⬆️
8.3	73.81% <91.66%> (+<0.01%)	⬆️
8.4	73.80% <91.66%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/API/Logs/LogRecord.php	100.00% <100.00%> (ø)
src/API/Trace/NonRecordingSpan.php	90.00% <100.00%> (ø)
src/Context/Propagation/MultiTextMapPropagator.php	100.00% <100.00%> (ø)
src/Contrib/Zipkin/Exporter.php	100.00% <ø> (ø)
src/SDK/Common/Dev/Compatibility/Util.php	100.00% <100.00%> (ø)
src/SDK/Logs/EventLogger.php	100.00% <ø> (ø)
src/SDK/Logs/LoggerProvider.php	100.00% <100.00%> (ø)
src/SDK/Metrics/MeterProvider.php	91.83% <ø> (-0.17%)	⬇️
src/SDK/Trace/Span.php	94.52% <100.00%> (ø)
src/SDK/Trace/TracerProvider.php	100.00% <ø> (ø)
... and 1 more

... and 4 files with indirect coverage changes

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