[Core] Execute features files without pickles

[mpkorstanje]

A feature without pickles is a valid feature file and should not throw
any exceptions when executed.

An empty feature file is also a valid feature file but we should throw
an exception when explicitly asked to execute it. It was probably a
mistake.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage remained the same at 85.975% when pulling 1b7b118 on fix-empty-feature-not-found into d840161 on master.