fix(firebase_core, web): return empty list from apps getter in WASM mode

[davidmigloz]

Summary

• Adds a proactive check in the apps getter to verify if the Firebase core module is loaded before accessing firebase.apps
• This avoids triggering a JavaScript exception when Firebase is not initialized, ensuring consistent behavior in both JS and WASM web builds

Fixes #17918

Changes

packages/firebase_core/firebase_core_web/lib/src/firebase_core_web.dart

Added a check using the existing pattern from _initializeCore():

if (globalContext.getProperty('firebase_core'.toJS) == null) {
  return [];
}

packages/firebase_core/firebase_core_web/test/firebase_core_web_exceptions_test.dart

Added a test to verify apps returns an empty list when Firebase is not initialized.

Test plan

• Added unit test for the apps getter
• All existing tests pass (flutter test --platform chrome)
• Manual verification in WASM mode:

  flutter run -d chrome --release --wasm
  # Access Firebase.apps before initializeApp() - should return []

🤖 Generated with Claude Code

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