Support [ID]! arrays in ID formatter.

[benmccallum]

There was a subtle bug in the existing code when handling arrays with nullable items when passed a null item.

Edit: I've since realised/remembered that arrays with nullable items is a really weird thing to do; but in any case; since it's allowed we should support it.

Details:
In the case of the is IEnumerable<string>, since an enumerable of string or string? both pass this check, if a null was passed in the array, it'd be passed to the IIdSerializer Deserialize method which would throw. This can be fixed by typing that to IEnumerable<string?> and then handling nulls appropriately, just adding null to the array.

In the case of the is IEnumerable<IdValue>, since IEnumerable<IdValue?> doesn't pass the check, my polymorphic id formatter (which intercepts and sets an IEnumerable<IdValue?> can never have it's value picked up and used, so that did need a new block of code to handle.

Testing:

new List<IdValue>() is IEnumerable<IdValue>
true
new List<IdValue>() is IEnumerable<IdValue?>
false
new List<IdValue?>() is IEnumerable<IdValue?>
true
new List<IdValue?>() is IEnumerable<IdValue>
false

new List<string?>() is IEnumerable<string>
true
new List<string?>() is IEnumerable<string?>
true

[michaelstaib]

Is this ready for review?

[benmccallum]

Hey @michaelstaib , it's ready now, sorry I thought I was done yesterday but had some more ideas to streamline this.

[sonarcloud]

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