Change the required flag to true for predefined-filter attribute in Elytron subsystem.

Elytron resource type configurable-sasl-server-factory has filters object-list attribute. Its attribute predefined-filter is an alternative for pattern-filter attribute. The pattern-filter has required flag set to true, but predefined-filter has it false. It results in possibility to add empty filter:

/subsystem=elytron/configurable-sasl-server-factory=t1:add(sasl-server-factory=elytron, filters=[{}])
{"outcome" => "success"}