Fix nullable annotations on IEquatable/IComparable constraints

[stephentoub]

Fixes #73457

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Issue Details

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Fixes #73457

Author:	stephentoub
Assignees:	-
Labels:	area-System.Memory
Milestone:	7.0.0

[stephentoub]

This actually found a pre-existing bug. This:

Console.WriteLine(new[] { "1", "2", "3" }.AsSpan().LastIndexOfAny("4", "5", null));

fails with:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at System.SpanHelpers.LastIndexOfAny[T](T& searchSpace, T value0, T value1, T value2, Int32 length)

I've fixed it as well in the second commit.

[stephentoub]

Failure is #73247

[adamsitnik]

LGTM, thank you @stephentoub !