fix: Set a non-null apiKey for demo projects in Dart

On Android the underlying Java SDK will throw an exception if the api key is
empty. Setting it to any arbitrary value is sufficient.

Author: emesterhazy

packages/firebase_core/firebase_core/lib/src/firebase.dart

@@ -68,7 +68,7 @@ class Firebase {
       // for backwards compatibility and simply override the user-provided
       // options.
       options = FirebaseOptions(
-        apiKey: '',
+        apiKey: '12345',
         appId: '1:1:$platformString:1',
         messagingSenderId: '',
         projectId: demoProjectId,

packages/firebase_core/firebase_core/test/firebase_core_test.dart

@@ -70,7 +70,7 @@ void main() {
     const String demoProjectId = 'demo-project-id';
     const String expectedName = demoProjectId;
     const FirebaseOptions expectedOptions = FirebaseOptions(
-      apiKey: '',
+      apiKey: '12345',
       // Flutter tests use android as the default platform.
       appId: '1:1:android:1',
       messagingSenderId: '',