Add support for nullable maps

{
 "id":1,
 "keys":{"m1":{"v7":{"k1":"k2"}},"m2":{"v1":"abc","v2":{"w1":0.24}}}
}
{
 "id":1,
 "keys":{"m1":{"v7":{"k1":"k3"}},"m2":{"v1":"abc","v2":{"w1":0.2}}}
}

select kvgen(keys) from `json_kvgenflatten/temp.json`;
+------------+
|   EXPR$0   |
+------------+
| [{"key":"m1","value":{"v7":{"k1":"k2"},"v2":{}}},{"key":"m2","value":{"v7":{},"v1":"abc","v2":{"w1":0.24}}}] |
| [{"key":"m1","value":{"v7":{"k1":"k3"},"v2":{}}},{"key":"m2","value":{"v7":{},"v1":"abc","v2":{"w1":0.2}}}] |
+------------+
2 rows selected (0.508 seconds)