Allows Optional lists with unique_items check

[mfulgo]

When using unique_items with an Optional[List[T]] field, the field validator would raise the following error if the field was not provided:

'NoneType' object is not iterable (type=type_error)

Updating the validator to return None in these cases avoids the issue.

Fixes #3957, fix #4050, fix #4119

[hramezani]

Thanks @mfulgo for this PR 👍

The fix LGTM

But, As it was a problem before and has not been introduced in 1.10.x I am not sure that we want to include it. IMHO, we can ignore this change as it's not critical and wait for the proper fix in V2.

@samuelcolvin What do you think?

[mfulgo]

I hear that, but I expect that folks will depend on 1.10.x for a while before being able to switch to V2. So bug fixes to this branch (and you could cherry pick this commit to 1.9.x) seem like a good thing to me.

In any case, thanks for y'alls efforts and consideration!

[raymondberg]

Thank you so much for this PR! We'd love to get this merged soon. Just adding some ❤️ and open source downstream customer 😄 .

[hramezani]

Please review

[samuelcolvin]

I hear that, but I expect that folks will depend on 1.10.x for a while before being able to switch to V2.

reluctantly, i think I agree with you.

I'm happy with this, @PrettyWood and @hramezani please confirm you're happy with this (or just merge if you are).

[hramezani]

I hear that, but I expect that folks will depend on 1.10.x for a while before being able to switch to V2.

reluctantly, i think I agree with you.

I'm happy with this, @PrettyWood and @hramezani please confirm you're happy with this (or just merge if you are).

Fine for me

[samuelcolvin]

thanks so much @mfulgo.

[WilliamDEdwards]

When is v1.10.3 expected to be released?

[samuelcolvin]

I'll try work on next week, any help with #4552 would be VERY MUCH appreciated and would help get the release out asap.

[WilliamDEdwards]

I'll try work on next week, any help with #4552 would be VERY MUCH appreciated and would help get the release out asap.

I can't help with code, so: #4631