Remove/mitigate the nullability of IdentityUserClaim's ClaimType and ClaimValue properties

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

The nullability of `IdentityUserClaim.ClaimType` and `IdentityUserClaim.ClaimValue` properties are misleading for users modifying the data directly. If a user does modify the data directly and sets a null value, `IdentityUserClaim.ToClaim()` will throw an `ArgumentNullException` because of the `Claim` class's constructor.

While I understand the intent of this is not to modify the data directly, there will be cases where some users will do it and they will assume that a null is valid value.

### Describe the solution you'd like

Unsure of which way to go on it and I think it needs a larger discussion.

One approach would be to remove the nullability for `IdentityUserClaim.ClaimType` and `IdentityUserClaim.ClaimValue` and force a migration on users

Another approach could be to change `IdentityUserClaim.ToClaim()` to return `Claim?`, which would imply if either `IdentityUserClaim.ClaimType` or`IdentityUserClaim.ClaimValue` is `null` then return `null`.

Another approach would be to document and recommend users adjust their DbContext accordingly, which would allow them to opt-in to the migration.

### Additional context

I can see that a similar issue https://github.com/dotnet/aspnetcore/issues/18679 exists and is closed, though I would like to see some guidance on how to approach the issue