[tests] Fix potential NullReferenceException in cecil-tests. (#16998)

Fixes this compiler warning:

    tests/cecil-tests/MarshalAsTest.cs(91,8): warning CS8602: Dereference of a possibly null reference.

tests/cecil-tests/MarshalAsTest.cs

@@ -88,6 +88,9 @@ namespace Cecil.Tests {
 			if (type is null && tr.FullName == "System.Runtime.InteropServices.NFloat")
 				return true;
 
+			if (type is null)
+				throw new Exception ($"Unable to resolve {tr.FullName}");
+
 			if (type.IsEnum)
 				return true;