gracefully handle unset properties

[becdetat]

Checkk.CheckGenericInvarian.FieldName should handle properties. Target.Body seems to be a member expression if set but a propertyExpression if not set:

class Person { public string Name{get;set;}}
var person1 = new Person { Name = "foo" };
Check.That(() => person1.Name).IsNotNullOrEmpty(); // works
var person2 = new Person();
Check.That(() => person2.Name).IsNotNullOrEmpty(); // throws an argument exception in CheckGenericInvariant.FieldName