CodeFix for Assert with null message causes ambiguous code.

[manfred-brands]

Assert.AreEqual(2d, 3d, null);

would generate

Assert.That(3d, Is.EqualTo(2d), null);

which does not compile

The call is ambiguous between the following methods or properties:
'Assert.That<TActual>(TActual, IResolveConstraint, FormattableString,
string, string)' and 'Assert.That<TActual>(TActual, IResolveConstraint,
Func<string>, string, string)'

[manfred-brands]

The same holds if an tolerance value is supplied:

Assert.AreEqual(2d, 3d, 0.1, null);
```

Which results in:
```
Assert.That(3d, Is.EqualTo(2d).Within(0.1), null);
```

[manfred-brands]

Fixed by #698