fix(pivot-wider): handle empty id columns

ibis/expr/types/relations.py

@@ -4432,7 +4432,13 @@ def pivot_wider(
                 key = names_sep.join(filter(None, key_components))
                 aggs[key] = arg if values_fill is None else arg.coalesce(values_fill)
 
-        return self.group_by(id_cols).aggregate(**aggs)
+        id_cols = id_cols.expand(self)
+
+        # no id columns, so do an ungrouped aggregation
+        if not id_cols:
+            return self.aggregate(**aggs)
+
+        return self.group_by(*id_cols).aggregate(**aggs)
 
     def relocate(
         self,