Fix some NRT on StringCollection

[lilinus]

StringCollection can contain null values. Fix annotations for AddRange and CopyTo to align with this.

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[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

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[eiriktsarpalis]

Looks reasonable based on the other APIs present in the collection. @stephentoub any objections?

[stephentoub]

This is correct and we should take it. Just note that it could introduce more warnings into a consumer.

[eiriktsarpalis]

How so? It's only input parameters and none of the changed methods are virtual.

[stephentoub]

How so? It's only input parameters and none of the changed methods are virtual.

I was thinking this would warn:
SharpLab. But it doesn't I believe for the same reason there's no compiler error with:
Base[] base = new Derived[1];
base[0] = new OtherDerived();
even though it'll fail at run-time.

[stephentoub]

Thanks