fix feature is nil case

[KazuCocoa]

I found a nil case error when I use v3.0.0.pre.beta.4 in my project.
When turnip loads blank files such as examples/blank.feature, then we can see the following error.

./lib/turnip/rspec.rb:72:in `run': undefined method `line' for nil:NilClass (NoMethodError)

To avoid this error, I insert return nil if the feature is nil case.

Could you review this ?

[gongo]

@KazuCocoa Thanks!!

[gongo]

I've released 3.0.0 beta.5 🎉