Catch .firstWhere error in homework_api.dart.

[Jonas-Sander]

Found this while trying to debug the integration tests.
It's nothing really that showed up in the CI from what I know but doesn't hurt to fix it I guess.
This is an error that shows up after a test case already failed. See below.

PS F:\Development\projects\sharezone-app\app> fvm flutter test integration_test --flavor dev --dart-define USER_1_EMAIL=**** --dart-define USER_1_PASSWORD=****
>> 
Running Gradle task 'assembleDevDebug'...                         152,5s
√  Built build\app\outputs\flutter-apk\app-dev-debug.apk.
Installing build\app\outputs\flutter-apk\app-dev-debug.apk...         4,0s
00:00 +0: (setUpAll)
00:00 +0: Authentication User should be able to sign in
00:00 +0 -1: Authentication User should be able to sign in
00:00 +0 -1: (setUpAll) [E]
  Bad state: No element
  dart:async                                                                                                   Stream.firstWhere
  package:sharezone/util/api/homework_api.dart 47:22                                                           new HomeworkGateway
  package:sharezone/util/api.dart 74:20                                                                        new SharezoneGateway._
  package:sharezone/util/api.dart 47:29                                                                        new SharezoneGateway
  package:sharezone/main/run_app.dart 140:26                                                                   initializeDependencies.<fn>
  dart:async                                                                                                   _AsyncStarStreamController.add
  package:firebase_auth_platform_interface/src/method_channel/method_channel_firebase_auth.dart                MethodChannelFirebaseAuth.authStateChanges
  ===== asynchronous gap ===========================
  dart:async                                                                                                   _ForwardingStream.listen
  package:sharezone/main/run_app.dart 137:30                                                                   initializeDependencies
  ===== asynchronous gap ===========================
  dart:async                                                                                                   _CustomZone.registerUnaryCallback
  package:sharezone/main/run_app.dart 94:3                                                                     initializeDependencies
  f:/Development/projects/sharezone-app/app/integration_test/app_test.dart 23:26                               main.<fn>
  ===== asynchronous gap ===========================
  package:stream_channel                                                                                       _GuaranteeSink.add
  c:/Users/jonas/AppData/Local/Temp/flutter_tools.d68523da/flutter_test_listener.e0221e09/listener.dart 53:22  main.<fn>

  This test failed after it had already completed. Make sure to use [expectAsync]
  or the [completes] matcher when testing async code.
  dart:async                                                                                                   Stream.firstWhere
  package:sharezone/util/api/homework_api.dart 47:22                                                           new HomeworkGateway
  package:sharezone/util/api.dart 74:20                                                                        new SharezoneGateway._
  package:sharezone/util/api.dart 47:29                                                                        new SharezoneGateway
  package:sharezone/main/run_app.dart 140:26                                                                   initializeDependencies.<fn>
  dart:async                                                                                                   _AsyncStarStreamController.add
  package:firebase_auth_platform_interface/src/method_channel/method_channel_firebase_auth.dart                MethodChannelFirebaseAuth.authStateChanges
  ===== asynchronous gap ===========================
  dart:async                                                                                                   _ForwardingStream.listen
  package:sharezone/main/run_app.dart 137:30                                                                   initializeDependencies
  ===== asynchronous gap ===========================
  dart:async                                                                                                   _CustomZone.registerUnaryCallback
  package:sharezone/main/run_app.dart 94:3                                                                     initializeDependencies
  f:/Development/projects/sharezone-app/app/integration_test/app_test.dart 23:26                               main.<fn>
  ===== asynchronous gap ===========================
<asynchronous suspension>
  User should be able to sign in
════════════════════════════════════════════════════════════════════════════════════════════════════
00:08 +0 -2: Authentication User should be able to sign in [E]
  Test failed. See exception logs above.
  The test description was: User should be able to sign in

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[nilsreichardt]

LGTM