Fix nullable cast (#46)

In a future version of the compiler, this cast is a warning (csc version 3.10, change introduce [here](dotnet/roslyn#48803))

This resolves the warning for this case.

Author: jmarolf

Sdk/AssertEqualityComparerAdapter.cs

@@ -31,12 +31,15 @@ public AssertEqualityComparerAdapter(IEqualityComparer<T> innerComparer)
 		/// <inheritdoc/>
 #if XUNIT_NULLABLE
 		public new bool Equals(object? x, object? y)
+		{
+			return innerComparer.Equals((T?)x, (T?)y);
+		}
 #else
 		public new bool Equals(object x, object y)
-#endif
 		{
 			return innerComparer.Equals((T)x, (T)y);
 		}
+#endif
 
 		/// <inheritdoc/>
 		public int GetHashCode(object obj)