Link field plugin doesn't properly filter out non-entity paths

Problem/Motivation

In the link field plugin we are using whatever comes out from $this->getTargetEntity($field_item) and assuming it's an entity info, but in some cases it will not. (A link to the path "internal:/blog" for instance will return NULL there.

Proposed resolution

Check that what the link item is pointing to is really an entity before trying to use it.

Remaining tasks

User interface changes

API changes

Data model changes