[lib] address feedback from D13098

Summary: accidentally landed D13098 without addressing a piece of feedback.

Test Plan: tested this change with rest of original stack to make sure cache worked as expected

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13139

lib/utils/fc-cache.js

@@ -141,7 +141,11 @@ class FCCache {
     }
 
     const { farcasterUser } = cacheResult;
-    if (typeof farcasterUser !== 'object' || farcasterUser instanceof Promise) {
+    if (
+      typeof farcasterUser !== 'object' ||
+      farcasterUser instanceof Promise ||
+      !farcasterUser
+    ) {
       return undefined;
     }