Bug 1893608 - [remote] Fix error this.postData is undefined in Networ…

…kRequest class r=webdriver-reviewers,Sasha

We do not have any test checking requests with postData, and due to a technical limitation we only have this field available in the
response events, so the error from my previous patch went unnoticed.

Differential Revision: https://phabricator.services.mozilla.com/D208737

remote/shared/NetworkRequest.sys.mjs

@@ -84,7 +84,7 @@ export class NetworkRequest {
   }
 
   get postDataSize() {
-    return this.#postData ? this.postData.size : 0;
+    return this.#postData ? this.#postData.size : 0;
   }
 
   get redirectCount() {