Fix empty relationship queries on SelectFilter with hasEmptyOption enabled

[AlexanderPoellmann]

Description

The SelectFilter injects an option with the key __empty to the top of the options when hasEmptyOption is enabled (integrating a relationship).

This causes an "Invalid text representation" error, as __empty is an invalid value for columns of type int / bigint ("id" = __empty) on pgsql.

In order to fix this I have added a static method getRelationshipQueryValues, which filters said __empty key from the (state) value string / values array. I've replaced the arrays $state['values'] and $values with this helper method only where a relationship is queried, so custom option arrays remain untouched.

In the apply method I have changed the query to check for an empty value / empty values, so the relationship scope would not be applied, as "id" = "" would result in yet another error.

For convenience I've also introduced a static property carrying the $emptyRelationshipOptionKey (__empty).

Visual changes

There are no visual changes.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.