Fix PHP 8.4 deprecations

[JeppeKnockaert]

Description

This PR removes implicitly nullable parameters because these result in deprecation warnings for PHP 8.4.

https://www.php.net/manual/en/migration84.deprecated.php#migration84.deprecated.core.implicitly-nullable-parameter

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 10.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 7.07%. Comparing base (c1cde2d) to head (a3e7f6a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ddtrace_php_api.stubs.php	0.00%	7 Missing ⚠️
src/DDTrace/OpenTelemetry/Span.php	0.00%	5 Missing ⚠️
src/DDTrace/OpenTracer/Tracer.php	0.00%	2 Missing ⚠️
src/DDTrace/OpenTracer1/Tracer.php	0.00%	2 Missing ⚠️
...grations/CodeIgniter/V2/CodeIgniterIntegration.php	0.00%	1 Missing ⚠️
src/DDTrace/OpenTelemetry/SpanBuilder.php	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (c1cde2d) and HEAD (a3e7f6a). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (c1cde2d)	HEAD (a3e7f6a)
tracer-php	11	6

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #2981       +/-   ##
============================================
- Coverage     74.82%   7.07%   -67.75%     
  Complexity     2741    2741               
============================================
  Files           110     110               
  Lines         10863   10863               
============================================
- Hits           8128     769     -7359     
- Misses         2735   10094     +7359     

Flag	Coverage Δ
tracer-php	7.07% <10.00%> (-67.75%)	⬇️

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

Files with missing lines	Coverage Δ
src/DDTrace/Integrations/AMQP/AMQPIntegration.php	0.00% <ø> (-98.45%)	⬇️
src/DDTrace/Integrations/Logs/LogsIntegration.php	0.00% <ø> (-93.29%)	⬇️
...egrations/WordPress/WordPressIntegrationLoader.php	0.00% <ø> (-80.60%)	⬇️
src/DDTrace/ScopeManager.php	84.84% <100.00%> (-15.16%)	⬇️
src/DDTrace/Tracer.php	55.40% <100.00%> (-27.03%)	⬇️
...grations/CodeIgniter/V2/CodeIgniterIntegration.php	0.00% <0.00%> (-50.00%)	⬇️
src/DDTrace/OpenTelemetry/SpanBuilder.php	0.00% <0.00%> (-93.85%)	⬇️
src/DDTrace/OpenTracer/Tracer.php	0.00% <0.00%> (ø)
src/DDTrace/OpenTracer1/Tracer.php	0.00% <0.00%> (-86.67%)	⬇️
src/DDTrace/OpenTelemetry/Span.php	0.00% <0.00%> (-85.43%)	⬇️
... and 1 more

... and 71 files with indirect coverage changes

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