fix NPE in updateMessagesData

[mahibi]

Exception java.lang.NullPointerException:
  at com.nextcloud.talk.chat.data.network.OfflineFirstChatRepository.updateMessagesData (OfflineFirstChatRepository.kt:1341)
  at com.nextcloud.talk.chat.data.network.OfflineFirstChatRepository.access$updateMessagesData (OfflineFirstChatRepository.kt:55)
  at com.nextcloud.talk.chat.data.network.OfflineFirstChatRepository$updateMessagesData$1.invokeSuspend (OfflineFirstChatRepository.kt:19)
  at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith (ContinuationImpl.kt:34)
  at kotlinx.coroutines.DispatchedTask.run (DispatchedTask.kt:98)
  at android.os.Handler.handleCallback (Handler.java:958)
  at android.os.Handler.dispatchMessage (Handler.java:99)
  at android.os.Looper.loopOnce (Looper.java:224)
  at android.os.Looper.loop (Looper.java:318)
  at android.app.ActivityThread.main (ActivityThread.java:8763)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:561)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1013)

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/30840816882/artifacts/8866919419
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
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