Fix the checksum type check

[Flamefire]

The None case was missed and due to the unrestricted tuple elem_type it may return valid for actually invalid entries. So restrict that beeing overly cautious so it may wrongly return invalid.
But in that case the conversion function will be called which can do more elaborate verification.

Add test checking for None in checksums.

Extracted from #4159, see https://github.com/easybuilders/easybuild-framework/pull/4159#issuecomment-1373429332

Also required for #4142 as the check now correctly handles a None value in the dict

Requires resolution of

• [ ] https://github.com/easybuilders/easybuild-docs/pull/104

[Flamefire]

Converted to draft to first specify how checksums especially "None" should work

[Flamefire]

Specifying as ready as the inability to specify alternate checksums in a dict blocks e.g. https://github.com/easybuilders/easybuild-easyconfigs/pull/19231

[Flamefire]

Can this (and https://github.com/easybuilders/easybuild-docs/pull/104) be addressed? It is also a dependency of https://github.com/easybuilders/easybuild-framework/pull/4159

[Flamefire]

Closed for https://github.com/easybuilders/easybuild-framework/pull/4578