fix: convert to str than filter

prevent empty strings from getting passed to mongodb
https://github.com/NodeBB/NodeBB/issues/13007#issuecomment-2560503754

test/database/hash.js

@@ -521,6 +521,7 @@ describe('Hash methods', () => {
 
 		it('should not error if fields is empty array', async () => {
 			await db.deleteObjectFields('someKey', []);
+			await db.deleteObjectField('someKey', []);
 		});
 
 		it('should not error if key is undefined', (done) => {