[lib] fix useFCNames

Summary: resolves https://linear.app/comm/issue/ENG-9337/fix-broken-usefcnames

Test Plan:
https://gist.github.com/vdhanan/9b6a003fb731f7e23fac54e8d594fdef

with patch, i was able to see my farcaster username logged

Reviewers: ashoat, will

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13387

lib/hooks/fc-cache.js

@@ -32,7 +32,7 @@ function useFCNames<T: ?BaseFCInfo>(
         if (farcasterUsername) {
           cachedResult = farcasterUsername;
         } else if (fid && fcCache) {
-          cachedResult = fcCache.getCachedFarcasterUserForFID(fid);
+          cachedResult = fcCache.getCachedFarcasterUserForFID(fid)?.username;
         }
         return {
           input: user,