fix: JSON().Len() and JSON().JQ().Len() on null objects

[Eun]

Problem

• Expect().Body().JSON().Len() is returning an error when the payload is null.
• Expect().Body().JSON().JQ().Len() is returning an error when the specified JQ expression returns null.

Solution

Both cases should return a len of 0.

Checklist

• Ran make test
• Review README.md go //ignore sections
• Added Changelog entry to README.md when this is a #major or #minor release.