fix: check if shraed files exist in ll_readshares

This may have been breaking the ability to see directories shared by
other users sometimes.

src/backend/src/filesystem/ll_operations/ll_readshares.js

@@ -79,6 +79,7 @@ class LLReadShares extends LLFilesystemOperation {
         }
 
         for ( const node of interm_results ) {
+            if ( ! await node.exists() ) continue;
             if ( ! await svc_acl.check(actor, node, 'see') ) continue;
             results.push(node);
         }