Suggest entities for linking only if there an empty feature in a target annotation to be linked

[Richard Eckart de Castilho]

Is your feature request related to a problem? Please describe.
The entity linker recommender requires a target annotation to be triggered and will then predict a concept to which the target annotation can be linked. Currently, when the target layer allows stacking, the linker will indefinitely propose concepts.

Describe the solution you'd like
By default, the linker should propose concepts only if there is a target annotation that has a non-empty target feature. An annotation with a filled target feature should then not trigger the linker.