refactor(localize): avoid computing source-maps in extraction unnecessarily (#40891)

Previously we were calling `updateSourceLocations()` as part of
`extractMessages()` for every file that was passed in, regardless of
whether any `$localize` tagged strings were to be found in the file.

This was very wasteful because it is non-trivial to compute the flattened
source-map for files if it is not needed.

PR Close #40891

packages/localize/src/tools/src/extract/extraction.ts

@@ -58,10 +58,10 @@ export class MessageExtractor {
         code: false,
         ast: false
       });
-    }
-    if (this.useSourceMaps) {
+      if (this.useSourceMaps && messages.length > 0) {
         this.updateSourceLocations(filename, sourceCode, messages);
       }
+    }
     return messages;
   }