UI: Don't save/overwrite browser docks if CEF hasn't loaded

OBS only populates the extraBrowserDocks list if `cef` is valid. By
saving docks regardless of whether `cef` is valid, this can result in
the user's custom docks being cleared unexpectedly.

UI/window-basic-main.cpp

@@ -4768,7 +4768,9 @@ void OBSBasic::closeEvent(QCloseEvent *event)
 			  saveState().toBase64().constData());
 
 #ifdef BROWSER_AVAILABLE
-	SaveExtraBrowserDocks();
+	if (cef)
+		SaveExtraBrowserDocks();
+
 	ClearExtraBrowserDocks();
 #endif