fix: use fanout

Author: Alan Shaw

packages/ipfs-unixfs-exporter/src/resolvers/unixfs-v1/content/hamt-sharded-directory.ts

@@ -1,3 +1,5 @@
+import { UnixFS } from 'ipfs-unixfs'
+import errCode from 'err-code'
 import { decode, type PBNode } from '@ipld/dag-pb'
 import errCode from 'err-code'
 import { UnixFS } from 'ipfs-unixfs'
@@ -33,7 +35,7 @@ async function * listDirectory (node: PBNode, path: string, resolve: Resolve, de
     throw errCode(err, 'ERR_NOT_UNIXFS')
   }
 
-  if (dir.fanout == null) {
+  if (!dir.fanout) {
     throw errCode(new Error('missing fanout'), 'ERR_NOT_UNIXFS')
   }