Allow null string message to be passed to Assert.That

[manfred-brands]

Fixes #4532

[stevenaw]

LGTM. Good catch @manfred-brands

[manfred-brands]

LGTM. Good catch @manfred-brands

I only noticed it because some my work's NUnit extensions didn't compile anymore with version 4.

[OsirisTerje]

@manfred-brands Awesome! Very good that you found this before real release. The more we manage to test the beta, the more likely it is we wont have any hotfixes.

[manfred-brands]

@OsirisTerje I'm still at the 'compile' phase. I thought I had the big repository all done, but then the tests failed with missing methods because the NUnitExtensions we have were still trying to resolve NUnit3 signatures of Assert.That. So next step is to update those libraries.

[OsirisTerje]

@manfred-brands Could be interesting if you wrote up your experiences, could be useful for creating a migration guide.