fix: Update update_span_duration return type

[PROFeNoM]

Description

Fixes #2585

Changes update_span_duration return type from null to false|null

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

Merging #2586 (4150886) into master (5d2c3cb) will decrease coverage by 1.15%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2586      +/-   ##
============================================
- Coverage     76.94%   75.79%   -1.15%     
  Complexity     2564     2564              
============================================
  Files           215      241      +26     
  Lines         23076    27048    +3972     
  Branches          0      976     +976     
============================================
+ Hits          17756    20502    +2746     
- Misses         5320     6026     +706     
- Partials          0      520     +520     

Flag	Coverage Δ
appsec-extension	69.13% <ø> (?)
tracer-extension	78.73% <ø> (ø)
tracer-php	74.73% <ø> (ø)

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Files	Coverage Δ
ext/ddtrace_arginfo.h	100.00% <ø> (ø)

... and 26 files with indirect coverage changes

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