runtime: ValueTuple throws for null comparer, unlike other BCL APIs

Continuing from https://github.com/dotnet/corefx/issues/18432.

You would expect this to succeed, but it throws NullReferenceException:

((IStructuralEquatable)(1, 2)).Equals((1, 2), null)

I’ll quite often take advantage of the fact that all BCL APIs use EqualityComparer<T>.Default when you pass null, and chain constructors and other methods with the parameter IEqualityComparer<T> comparer = null. If my own constructor or extension method takes IEqualityComparer<T> comparer = null, I assume that I can pass that into the BCL method. It’s not intuitive to make it the call site’s responsibility to check for null and pass EqualityComparer<object>.Default or call one or the other BCL overload depending whether comparer = null.

It’s not critical since the workaround is to pass comparer ?? EqualityComparer<object>.Default instead of comparer. This is an API gotcha that may go unnoticed until code is in the field though.

ValueTuple<*>.IStructuralEquatable.Equals and ValueTuple<*>.IStructuralComparable.CompareTo have no null comparer check. If it followed the convention set by all other BCL methods, it would use EqualityComparer<object>.Default if you pass null.

Array.IStructuralEquatable.Equals and Array.IStructuralComparable.CompareTo, and Tuple<*>.IStructuralEquatable.Equals and Tuple<*>.IStructuralComparable.CompareTo have the same problem. They are in coreclr. Should I open an issue over there?