fix: only send library association on create when libraryIds is defined

Guard the createUser library association call with
libraryIds !== undefined, matching the updateUser pattern. This prevents
accidentally clearing backend default library assignments when the form
field hasn't been initialized yet.

ui/src/dataProvider/wrapperDataProvider.js

@@ -114,7 +114,7 @@ const createUser = async (params) => {
   const userId = userResponse.data.id
 
   // Then set library associations for non-admin users
-  if (!userData.isAdmin) {
+  if (!userData.isAdmin && libraryIds !== undefined) {
     await handleUserLibraryAssociation(userId, libraryIds)
   }