Improve AreSame error messages in case of null values

[ygeerts]

Summary

Currently the AreSame overload methods produce an AssertFailedException when either expected or actual are null and the other one isn't, but these messages do not mention anything about null values.

Example

string? expected = "test";
string? actual = null;
Assert.AreSame(expected, actual);

Actual result:

Assert.AreSame failed. 'expected' expression: 'expected', 'actual' expression: 'actual'.

Proposed result:

Assert.AreSame failed. 'expected' expression: 'expected', 'actual' expression: 'actual'. 'actual' was null.

[Youssef1313]

You still won't get good messages for cases like:

var expected = new string("test");
var actual = new string("test");
Assert.AreSame(expected, actual);

In that case, the object reference is different even if the string representation is the same.

I'm not sure if it's worth adjusting the message for this very niche case.

[drieseng]

@Youssef1313, in that case you'd first have to do a Assert.IsNotNull on the actual value to get a better idea on why exactly the subsequent Assert.AreSame failed. Haven't checked what message is produced by Assert.AreSame when actual and expected are not of the same type. Would be nice to have a distinct message for that scenario as well.

[Evangelink]

It seems it doesn't complexify much the code (see #7426) and adds some value for null cases. I agree with @Youssef1313 that the default message would still be not so helpful for scenarios like the string but overall that's IMO better experience than current so I'd vote for improving it