fluentassertions: Missing null checks

I’m playing around with nullable reference types in this branch. So far I’ve found several places that could benefit from an extra null check.

I’ll try to keep the branch in sync with master as the null checks are merged.

ReferenceTypeAssertions
	BeAssignableTo
	BeAssignableTo<T>
		null Subject

ComparableTypeAssertions
	Be
		null Subject
	
NumericAssertions
	BeOfType
	NotBeOfType
		null Subject
		
ActionAssertions
	NotThrow
	InvokeSubjectWithInterception (public methods that calls it)
		null Subject
		
AsyncFunctionAssertions
	NotThrow
	NotThrowAsync
	NotThrow<TException>
	NotThrowAsync<TException>
	InvokeSubjectWithInterception (public methods that calls it)
	InvokeSubjectWithInterceptionAsync (public methods that calls it)
		null Subject
		
FunctionAssertions
	NotThrow
	NotThrow<TException>
	InvokeSubjectWithInterception (public methods that calls it)
	NotThrowAfter
		null Subject
		
AssemblyAssertions
	DefineType
		null Subject
	Reference
	NotReference
		null Subject
		null assembly
		
ExecutionTimeAssertions
	ctor
		null action
		null Func<T>