Handle StatResult.Code is empty string

[DenitsaTH]

Description

This PR handles the case when StatResult.Code is an empty string. The following changes are introduced:

• An additional Ncode field is added to StatResult, corresponding to code in JSON.
• Empty string Code is handled in a custom json.Unmarshal method for Ncode. In case of empty string, 0 is assigned to Code.
• A function to assign value of Ncode to Code is added.

Motivation and Context

This fixes the issue of Go client not being able to handle codeless errors.

How Has This Been Tested?

TestEmptyResponseCode was added to test the behaviour with empty string Code.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)