Defend against nil bundle identifiers (#3188)

This fixes #3184, where under some circumstances some bundles have no
CFBundleIdentifier in their Info.plist.

Author: wilhuff

Firestore/core/src/firebase/firestore/remote/grpc_root_certificate_finder_apple.mm

@@ -37,6 +37,18 @@
 
 namespace {
 
+std::string DebugBundleIdentifier(NSBundle* _Nullable bundle) {
+  if (!bundle) return "(nil bundle)";
+
+  NSString* identifier = [bundle bundleIdentifier];
+  if (!identifier) {
+    identifier = [bundle bundlePath];
+  }
+
+  return identifier ? util::MakeString(identifier)
+                    : "(bundle with no identifier)";
+}
+
 /**
  * Finds the roots.pem certificate file in the given resource bundle and logs
  * the outcome.
@@ -53,10 +65,10 @@
   if (util::LogIsDebugEnabled()) {
     std::string message =
         absl::StrCat("roots.pem ", path ? "found " : "not found ", "in bundle ",
-                     util::MakeString([bundle bundleIdentifier]));
+                     DebugBundleIdentifier(bundle));
     if (parent) {
-      absl::StrAppend(&message, " (in parent ",
-                      util::MakeString([parent bundleIdentifier]), ")");
+      absl::StrAppend(&message, " (in parent ", DebugBundleIdentifier(parent),
+                      ")");
     }
     LOG_DEBUG("%s", message);
   }