Add NoNullValues validators

[jar-b]

Adds a new NoNullValues validator to the listvalidator and setvalidator packages.

Closes #245

[jar-b]

I'll hold off on a CHANGELOG entry until it's determined whether this will be accepted.

[austinvalle]

Hey there @jar-b 👋🏻 , thanks for raising the PR + issue.

This validator seems useful and I'm of the mind that moving more of the generic validation logic towards Go modules like this is a good thing 👍🏻

Couple initial thoughts:

• Did you have any other thoughts on potential names for the validator? Initially NoNullValues didn't register what the intent is to me. I don't have a super strong opinion on it as I don't really have a slam dunk alternative 😆. Would something like NoNullElements still make sense to you? Maybe @SBGoods or @bbasata have some thoughts.
  • That kind of manifests in some of the error messaging. i.e., the collection itself is not a null value, but one of the elements is a null value.
• Once we're on the same page with the name, would you mind implementing a similar validator for mapvalidator since it's also a collection?

I have some comments that are more nits about the description and error messages, but I'll save it for once we decide on the name

[austinvalle]

Ah, I missed your comment in the issue about the existing UniqueValues validator (as well as the ValueListsAre* validators 😄). Now I'm more on-board with your initial name.... consistency is probably more important here 🤔

[jar-b]

Thanks for taking a look at this! I added a map validator using the same NoNullValues naming convention, but I have no issues changing it to whatever you all think best describes the functionality.

[austinvalle]

Thanks again for the contribution @jar-b! I have some minor nits on the documentation and we'll want a changelog for this as well.

I wanted to release this along side another PR sometime later today, so I'm just going to push the changes/changelog to your branch and merge/release, thanks!