Adding some adjustments

| Q             | A
| ------------- | ---
| Bug fix?      | CS fixes/removing useless method
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | No need
It's done by `willReturn()` anyway, so it can be ommited. It's usefull only when you expect call but not returned value.
```php
$collection->add($object);
$collection->add($object)->shouldBeCalled();
# vs
if ($collection->isEmpty()) {}
$collection->isEmpty($object)->willReturn(false);