fix variable check in xcode_backend.sh (#24368)

* fix test in xcode_backend.sh

* -n

* fix devicelab test

* uncomment good code, check for \"\"

dev/devicelab/bin/tasks/module_test_ios.dart

@@ -154,8 +154,8 @@ Future<void> main() async {
             'Debug',
             'CODE_SIGNING_ALLOWED=NO',
             'CODE_SIGNING_REQUIRED=NO',
-            'CODE_SIGNING_IDENTITY=""',
-            'EXPANDED_CODE_SIGN_IDENTITY=""',
+            'CODE_SIGN_IDENTITY=',
+            'EXPANDED_CODE_SIGN_IDENTITY=',
             'CONFIGURATION_BUILD_DIR=${tempDir.path}',
           ],
         );

packages/flutter_tools/bin/xcode_backend.sh

@@ -367,7 +367,7 @@ EmbedFlutterFrameworks() {
 
   # Sign the binaries we moved.
   local identity="${EXPANDED_CODE_SIGN_IDENTITY_NAME:-$CODE_SIGN_IDENTITY}"
-  if (${identity} != ""); then
+  if [[ -n "$identity" && "$identity" != "\"\"" ]]; then
     RunCommand codesign --force --verbose --sign "${identity}" -- "${xcode_frameworks_dir}/App.framework/App"
     RunCommand codesign --force --verbose --sign "${identity}" -- "${xcode_frameworks_dir}/Flutter.framework/Flutter"
   fi