Fix MultiDB Regression (#735)

Author: tom-andersen

src/main/java/com/google/firebase/cloud/FirestoreClient.java

@@ -79,7 +79,8 @@ public static Firestore getFirestore() {
    */
   @NonNull
   public static Firestore getFirestore(FirebaseApp app) {
-    return getFirestore(app, ImplFirebaseTrampolines.getFirestoreOptions(app).getDatabaseId());
+    final FirestoreOptions firestoreOptions = ImplFirebaseTrampolines.getFirestoreOptions(app);
+    return getFirestore(app, firestoreOptions == null ? null : firestoreOptions.getDatabaseId());
   }
 
   /**

src/test/java/com/google/firebase/cloud/FirestoreClientTest.java

@@ -79,6 +79,20 @@ public void testServiceAccountProjectId() throws IOException {
     assertNotSame(firestore1, firestore2);
   }
 
+  @Test
+  public void testAbsentFirestoreOptions() throws Exception {
+    String projectId = "test-proj";
+    FirebaseOptions options = FirebaseOptions.builder()
+        .setCredentials(GoogleCredentials.fromStream(ServiceAccount.EDITOR.asStream()))
+        .setProjectId(projectId)
+        .build();
+
+    FirebaseApp.initializeApp(options);
+    Firestore firestore = FirestoreClient.getFirestore();
+    assertEquals(projectId, firestore.getOptions().getProjectId());
+    assertEquals("(default)", firestore.getOptions().getDatabaseId());
+  }
+
   @Test
   public void testFirestoreOptions() throws IOException {
     final String databaseId = "databaseIdInTestFirestoreOptions";