Fix missing function definition in FNullSentrySubsystem

[tustanivsky]

This PR fixes a plugin packaging issue caused by a missing IsUserConsentRequired function definition in FNullSentrySubsystem. The issue occurs when targeting platforms where the Sentry plugin is not enabled (e.g., when packaging for a game console without the corresponding plugin extension installed)

Related to:

• Expose user consent API #1139

Closes #1171

[github-actions]

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- Fix missing function definition in `FNullSentrySubsystem` ([#1172](https://github.com/getsentry/sentry-unreal/pull/1172))

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