Substitute.For and Substitute.ForPartsOf no longer allow null parameters #978

Describe the bug
When upgrading to NSubstitute version 6 I can no longer pass null parameters to Substitute.For and Substitute.ForPartsOf even though the abstract class constructor explicitely allows null values

To Reproduce

• Create an abstract class with a constructor taking a nullable string.
• Create a Substitute using Substitute.ForPartsOf(null);

Expected behaviour
I should be allowed to pass in null values when the receiving parameter allows null values

Environment:

• NSubstitute version: 6.0.0

References
Relates to #973