Assert.Throws<ArgumentException>(paramName:null, Action) throws ArgumentNullException

[ericstj]

Regression introduced here:
xunit/assert.xunit@726e482#diff-829bc159adac0f20073a5979fa4b4db6d7586a249058328199b486cf26b01070R189

Observed in xUnit 2.4.2-pre.13

For better or worse, we have framework code that doesn't specify a parameter name for some ArgumentExceptions. EG: https://github.com/dotnet/runtime/blob/57bfe474518ab5b7cfe6bf7424a79ce3af9d6657/src/libraries/System.Drawing.Common/tests/FontFamilyTests.cs#L63-L64

Specifying null for parameterName used to work for testing this scenario but now it throws in the Assert library. Can we bring back the old behavior?

[ericstj]

I'll add that we can workaround this by using Assert.Throws<ArgumentException>(Action) but we'd actually like to Assert a null parameter name, since it's clear that we didn't forget to test it. Today we'll use our own wrapper to do this, but I wanted to raise this breaking change of behavior.

[bradwilson]

This was probably just overly zealous on my part when I went through and introduced all the nullability. I probably assumed paramName was required in the exception.