Nullability: RuntimeHelpers.GetHashCode should accept null obj input

[GrabYourPitchforks]

Changes RuntimeHelpers.GetHashCode(object) to RuntimeHelpers.GetHashCode(object?). Both coreclr and mono already support passing null as an argument to this method.

runtime/src/mono/mono/metadata/monitor.c

Lines 565 to 573 in cd622cd

	int
	mono_object_hash_internal (MonoObject* obj)
	{
	#ifdef HAVE_MOVING_COLLECTOR
	LockWord lw;
	unsigned int hash;
	if (!obj)
	return 0;

runtime/src/coreclr/src/classlibnative/bcltype/objectnative.cpp

Lines 85 to 99 in 8e60385

	// Note that we obtain a sync block index without actually building a sync block.
	// That's because a lot of objects are hashed, without requiring support for
	FCIMPL1(INT32, ObjectNative::GetHashCode, Object* obj) {
	CONTRACTL
	{
	FCALL_CHECK;
	INJECT_FAULT(FCThrow(kOutOfMemoryException););
	}
	CONTRACTL_END;
	VALIDATEOBJECT(obj);
	if (obj == 0)
	return 0;

And we already have a unit test for it.

runtime/src/libraries/System.Runtime/tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs

Lines 35 to 36 in 8e60385

	int h5 = RuntimeHelpers.GetHashCode(null);
	Assert.Equal(0, h5);

[stephentoub]

Thanks