gh-612: handle NoneType in get_namespace

[paddyroddy]

Description

Make sure that the get_namespace method can be used even if one of the inputs is None.

I've also simplified the logic, so have changed not all(== to any(!=.

Closes: #612

Refs: #606

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[Saransh-cpp]

Thanks, @paddyroddy! This looks good to me!

[paddyroddy]

Merging as needed for #597