[OpenSIPS-Devel] [OpenSIPS/opensips] d59d43: Fix strange `"error":null` JSON node

[Bogdan Andrei IANCU]

  Branch: refs/heads/3.2
  Home:   https://github.com/OpenSIPS/opensips
  Commit: d59d43370a1630752805680f51340859b20fd07f
      https://github.com/OpenSIPS/opensips/commit/d59d43370a1630752805680f51340859b20fd07f
  Author: Bogdan-Andrei Iancu <bogdan at opensips.org>
  Date:   2023-04-10 (Mon, 10 Apr 2023)

  Changed paths:
    M modules/jsonrpc/jsonrpc.c

  Log Message:
  -----------
  Fix strange `"error":null` JSON node

An `error` node with NULL value does not make much of a sense, but let's try to handle it.
Closes #3047

(cherry picked from commit 1a8f8e47bf3badc68927fbb573b1e78c596d2055)