QmlDesigner: Avoid UniqueName::generateId() crash when id is empty

Change-Id: I07099c143db2317a0275b56a8300bdccf959b6c0
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/qmldesigner/designercore/uniquename.cpp

@@ -147,6 +147,9 @@ QString generatePath(const QString &path)
  */
 QString generateId(const QString &id, std::function<bool(const QString &)> predicate)
 {
+    if (id.isEmpty())
+        return {};
+
     // remove non word (non A-Z, a-z, 0-9) or space characters
     QString newId = id.trimmed();