[php-*] Explicitly declare nullable parameters

[JulianVennen]

PR #20243 fixed some deprecation warnings from PHP 8.4 in the PHP generator. This PR applies the same fixes to the other PHP clients.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

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  Commit all changed files.
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