Fix HashSet.TryGetValue throwing for null with a StringComparer

[SimonCropp]

Bug

The HashSet<T>.TryGetValue polyfill (active below net472) called comparer.GetHashCode on the lookup value and on every element of the set. HashSet<T> itself never passes null to the comparer's GetHashCode, so with a comparer such as StringComparer.OrdinalIgnoreCase the polyfill threw ArgumentNullException when the lookup value was null, or when the set contained null. It was also O(n) with a hash computed per element, even for a value that is not in the set.

Fix

Use Contains first, which is a hashed lookup and handles null the way the set does. Only when the value is present, scan for the stored instance using comparer.Equals alone (which is what the set itself calls for a null entry).

Test

TryGetValue_NullWithStringComparer uses an OrdinalIgnoreCase set containing null, and checks a case-insensitive hit returns the stored instance, a null lookup succeeds, a miss returns false, and a null lookup on a set without null returns false. It threw ArgumentNullException on net462 before the fix, and passes on net462 (polyfill), net48 and net10.0 (BCL).