Add the possibility to explicitly filter/include Entities that do not have any value for the configured field to the org.apache.stanbol.entityhub.indexing.core.processor.FieldValueFilter
1. if the values parameter is not present do not longer throw an Exception, but use this to filter all Entities with no value
2. use "values=" and "values=null" to filter all entities that do have values for the configured field
All other combinations are already supported by the current implementation
NOTE: that this change will not change the behavior of any currently valid configuration as the newly added functionality would trigger IllegalArgumentExceptions with the old implementation.