Add undefined check for accessing account id

If uploader is undefined(as is the case discovered when editing a
file) then accessing _account_id thrown an error

Change-Id: I8503dff738e307df45dcc0032491abedcfbe0971
diff --git a/polygerrit-ui/app/utils/change-util.ts b/polygerrit-ui/app/utils/change-util.ts
index be5c449..094aa3b 100644
--- a/polygerrit-ui/app/utils/change-util.ts
+++ b/polygerrit-ui/app/utils/change-util.ts
@@ -186,7 +186,7 @@
 export function isUploader(change?: ChangeInfo, account?: AccountInfo) {
   if (!change || !account) return false;
   const rev = getCurrentRevision(change);
-  return rev?.uploader._account_id === account._account_id;
+  return rev?.uploader?._account_id === account._account_id;
 }
 
 export function isInvolved(change?: ChangeInfo, account?: AccountInfo) {