Fixups in set_value_to_json_map

[xzfc]

This PR updates the behavior set_value_to_json_map in the following ways:

1. Do not extend arrays with null values.
   So, set_value_to_json_map("a[5]", json!{}, x) will no longer produce {"a": [null, null, null, null, null, x]}. Instead, it will produce { "a": [] }.
   Reason: while this behavior is consistent with object fields, it enables an amplification attack, e.g. path=a[9223372036854775808] could crash the server.

2. Replace non-object values with an object.
   E.g. set_value_to_json_map("a", json!{"a": 0}, json!{"x": 1}) will produce {"a": {"x": 1}} instead of {"a": 0}.