Use the default option's project id if not provided.

FIS on iOS (and soon Android) requires that the project id be set, which prevents Apps from being created without it.  To prevent potential problems that developers might face, fall back to the default App's project id when creating additional apps.

PiperOrigin-RevId: 295176371

Author: a-maurice

app/src/app_options.cc

@@ -179,9 +179,9 @@ bool AppOptions::PopulateRequiredWithDefaults(
     JNIEnv* jni_env, jobject activity
 #endif  // FIREBASE_PLATFORM_ANDROID
 ) {
-  // Populate App ID and API key from the default options if they're not
-  // specified.
-  if (app_id_.empty() || api_key_.empty()) {
+  // Populate App ID, API key, and Project ID from the default options if
+  // they're not specified.
+  if (app_id_.empty() || api_key_.empty() || project_id_.empty()) {
     AppOptions default_options;
     if (AppOptions::LoadDefault(&default_options
 #if FIREBASE_PLATFORM_ANDROID
@@ -190,13 +190,15 @@ bool AppOptions::PopulateRequiredWithDefaults(
                                 )) {
       if (app_id_.empty()) app_id_ = default_options.app_id_;
       if (api_key_.empty()) api_key_ = default_options.api_key_;
+      if (project_id_.empty()) project_id_ = default_options.project_id_;
     } else {
       LogError("Failed to load default options when attempting to populate "
                "missing fields");
     }
   }
-  if (app_id_.empty() || api_key_.empty()) {
-    LogError("App ID and API key must be specified in App options.");
+  if (app_id_.empty() || api_key_.empty() || project_id_.empty()) {
+    LogError(
+        "App ID, API key, and Project ID must be specified in App options.");
     return false;
   }
   return true;