EntityhubDereferenceEngine fails if an empty String is parsed as dereference field

If [""] is parsed as dereference fields the engine creates a FieldMapper instance but will not add a single mapping as "" is not valid. As a result the FieldMapper will reject all fields and therefore causes no data to be dereferenced wile this config should rather dereference all fields of the Entity.

The same Issue can be also achieved by parsing any other invalid field mapping, but an empty configuration is the most likely reason causing this.

To fix this the activation method needs to check if a valid FieldMapping was parsed and if not do not create a FieldMapper instance.

As a Workaround users should parse ["*"] instead of an empty string.