SafeHandle marshaler should allow null handles

[stephentoub]

Today, the SafeHandle marshaler prohibits null SafeHandle instances:

runtime/src/coreclr/src/System.Private.CoreLib/src/System/StubHelpers.cs

Lines 1251 to 1256 in e489111

	internal static IntPtr SafeHandleAddRef(SafeHandle pHandle, ref bool success)
	{
	if (pHandle == null)
	{
	ThrowHelper.ThrowArgumentNullException(ExceptionArgument.pHandle, ExceptionResource.ArgumentNull_SafeHandle);
	}

but there are native methods that let you pass a null pointer to mean "I don't have one". To call such methods, you then either need to use an IntPtr directly or have multiple P/Invoke definitions (one for when you have a non-null SafeHandle and one for when you have null) or create a throw-away safe handle instance that contains IntPtr.Zero. It's not clear to me why the marshaler prohibits null; it could just marshal it as null / zero.

[jeffschwMSFT]

@AaronRobinsonMSFT

[AaronRobinsonMSFT]

We should talk about this for .NET 6. The built-in SafeHandle marshaller is a bit aggressive here but at a minimum the new source generation experiment needs to handle this with minimal fanfare.

/cc @elinor-fung @jkoritzinsky

[AaronRobinsonMSFT]

Reopening due to #48193