remove redudnant cast to IReadOnlyList and add a not null ckeck in AssemblyEnumeratorTests

[SimonCropp]

…semblyEnumeratorTests

[Youssef1313]

This assert is failing. It's because the mocking two lines above is throwing ReflectionTypeLoadException passing null array.

[Youssef1313]

NOTE: The failure is .NET Framework only, due to difference in ReflectionTypeLoadException implementation between .NET Core and .NET Framework.

When the constructor in .NET Core receives null, it sets the property to empty array:

https://github.com/dotnet/runtime/blob/0e94dca5b3203cdb9077dd543d87f1a69ff7eb40/src/libraries/System.Private.CoreLib/src/System/Reflection/ReflectionTypeLoadException.cs#L23

This is not the case for .NET Framework.