fix empty feature extractor #950
Description
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