[release/9.0] Initialize managed and native values in the ICustomMars…

…haler marshaler to null when the other is null. (#109096)

* Initialize managed and native values to null when the other is null.

* Update StubHelpers.cs

---------

Co-authored-by: Jeremy Koritzinsky <jekoritz@microsoft.com>
Co-authored-by: Jeremy Koritzinsky <jkoritzinsky@gmail.com>

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

@@ -807,17 +807,19 @@ internal static unsafe void ConvertContentsToNative(ICustomMarshaler marshaler,
             // COMPAT: We never pass null to MarshalManagedToNative.
             if (pManagedHome is null)
             {
+                *pNativeHome = IntPtr.Zero;
                 return;
             }
 
             *pNativeHome = marshaler.MarshalManagedToNative(pManagedHome);
         }
 
-        internal static void ConvertContentsToManaged(ICustomMarshaler marshaler, ref object pManagedHome, IntPtr* pNativeHome)
+        internal static void ConvertContentsToManaged(ICustomMarshaler marshaler, ref object? pManagedHome, IntPtr* pNativeHome)
         {
             // COMPAT: We never pass null to MarshalNativeToManaged.
             if (*pNativeHome == IntPtr.Zero)
             {
+                pManagedHome = null;
                 return;
             }