A `NullRelation` should represent nothing. 

Closes #15176.
I don't think so...

```ruby
def assert_operator o1, op, o2 = UNDEFINED, msg = nil
  return assert_predicate o1, op, msg if UNDEFINED == o2
  msg = message(msg) { "Expected #{mu_pp(o1)} to be #{op} #{mu_pp(o2)}" }
  assert o1.__send__(op, o2), msg
end

# => 0.< Comments.count
```