Constraint Streams: improve coverage of Joiners

Joiners have suboptimal code coverage, varying across streams of different arity. The default value of Quality gate is 80%, Joiners are very often below this threshold.
For more details about existing coverage, please have a look there [1]

[1] https://sonarcloud.io/component_measures?id=org.optaplanner%3Aoptaplanner&metric=coverage&selected=org.optaplanner%3Aoptaplanner%3Aoptaplanner-core%2Fsrc%2Fmain%2Fjava%2Forg%2Foptaplanner%2Fcore%2Fimpl%2Fscore%2Fstream&view=treemap