Improve IsExactInstanceOfType error message for NULL value

[drieseng]

Summary

Currently the IsExactInstanceOfType overloads produce an AssertFailedException when the input value is NULL, but these messages are not very clear:

Example

string? input = null;
Assert.IsExactInstanceOfType<string>(input);

Actual result:

Assert.IsExactInstanceOfType failed. 'value' expression: 'input'.

Proposed result:

Assert.IsExactInstanceOfType failed. 'value' expression: 'input'. 'value' was NULL.

[drieseng]

To align with #7421, use <null> in the failure messages instead of NULL.