NPE when multi-value feature has no value

[reckart]

Describe the bug
There can be an NPE when a multi-value feature has no value.

Screenshots

java.lang.NullPointerException: Cannot invoke "java.util.List.stream()" because "handles" is null
    at de.tudarmstadt.ukp.inception.ui.kb.feature.MultiValueConceptFeatureSupport.getLazyDetails(MultiValueConceptFeatureSupport.java:343) ~[inception-ui-kb-27.5-SNAPSHOT.jar!/:?]
    at de.tudarmstadt.ukp.inception.rendering.Renderer.getLazyDetails(Renderer.java:119) ~[inception-api-schema-27.5-SNAPSHOT.jar!/:?]
    at de.tudarmstadt.ukp.clarin.webanno.api.annotation.rendering.Renderer_ImplBase.renderLazyDetails(Renderer_ImplBase.java:150) ~[inception-api-annotation-27.5-SNAPSHOT.jar!/:?]

Please complete the following information:

• Version and build ID: 27.5