Re-add upload-symbols warning for invalid dSYMs (#7801)

Author: samedson

Crashlytics/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- [changed] Added a warning to upload-symbols when it detects a dSYM without any symbols.
+
 # v7.9.0
 - [changed] Updated Firebase pod to allow iOS 9 installation via `pod 'Firebase/Crashlytics'`