I understand the confusion about ```Given``` vs ```When``` when only 1 is needed
However:
- ```Given``` is for preparing the system for the test
- ```When``` is the actual test (the action)
- ```Then``` is asserting that the test went as expected

So there if you don't have to prepare any thing for the test, you have no need of a ```Given``` step, but ```When``` and ```Then``` **must** exist on the scenario. 
Doesn't make sense to verify/assert the preparing phase