fix empty feature extractor

[robstoll]

Affected Version: 0.16.0
API (fluent-en_GB, infix-en_GB): both
Platform (jvm, js): all
Extension (none, kotlin 1.3): none

How to reproduce the problem

See https://github.com/robstoll/atrium#ex-own-compose-4. Now that we report the expectation the user wanted to create if the iterable were not empty, we can see that we create an empty feature assertion additionally which shouldn't be created.

Describe the bug

The feature age is listed twice

Expected behaviour

The feature age in the example should be shown only once in reporting