Handle empty feature files

An totally empty feature file should not result in an undefined method error, see https://github.com/cucumber/cucumber/issues/771.

Running an empty feature file in Cucumber v1.3.x results in:
```
0 scenarios
0 steps
0m0.000s
```
So that should be the behaviour also of Cucumber v2.0.

Closes https://github.com/cucumber/cucumber/issues/771