[CVariant] Fix empty() for Null-CVariant

[neo1973]

Description

CVariant::empty() has to return true for a Null-CVariant. This accidentally broke in #23114 728b396. This PR restores the correct bahaviour and extends the unit test for that case.

The problem was first observed in xbmc/Official-Kodi-Remote-iOS#877.

Motivation and context

Restore the correct behaviour.

How has this been tested?

The curl request in xbmc/Official-Kodi-Remote-iOS#877 (comment) creates the expected output. Also there is a better unit test now.

What is the effect on users?

This could cause issues in many places.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[garbear]

Looks good. Thanks for including test cases.

[howie-f]

thanks for the quick fix. 👍