UI: Sync 'Copy Filters' enabled state in the Source context menu

This matches the behaviour of Scenes and the Audio Mixer.

UI/window-basic-main.cpp

@@ -5024,9 +5024,8 @@ void OBSBasic::CreateSourcePopupMenu(int idx, bool preview)
 
 		ui->actionCopyFilters->setEnabled(true);
 		ui->actionCopySource->setEnabled(true);
-	} else {
-		ui->actionPasteFilters->setEnabled(false);
 	}
+	ui->actionPasteFilters->setEnabled(copyFiltersString && idx != -1);
 
 	popup.exec(QCursor::pos());
 }