Null checks on delegates

[jnyrup]

When asserting on delegates we do not check if the delegate is non-null before invoking it.

FailIfSubjectIsAsyncVoid is changed so it does not run for AsyncFunctionAssertions via a new CanHandleAsync property.
The reason it worked before, was that ExecuteInDefaultSynchronizationContext wrapped an async Task method in a Task method.

[dennisdoomen]

Isn't there a reverse version of subject is null?

[jnyrup]

The only exact opposite of p is null is !(p is null).
The closest syntax without a negation operator is p is object.
The difference between the two is that:

• is null only works on reference types as value types cannot be null.
• is object also works on value types, as every types inherits object, but is always true and gives a CS0183 warning.

In C# 8 with recursive patterns, you could even write
p is {} x as a shorthand for p is MyClass x.

But I would be happy to replace as many ReferenceEquals(p, null) with p is object/p is null in the code base.