[DMN] COLLECT hit policy with `alwaysUseArraysForDmnMultiHitPolicies` returns null/value instead of array

[martin-grofcik]

Description BPMN/CMMN dmn service task for COLLECT hit policy returns (protected boolean alwaysUseArraysForDmnMultiHitPolicies = true;):

hits	output
none hit	null
one item hit	itemValue
multiple hits	array of values

The issue is also related to #2286 test: https://github.com/martin-grofcik/flowable-engine/commit/704b6bb1bec7055c708c525880ab2097bc104a55

Expected behavior BPMN/CMMN dmn service task for COLLECT hit policy returns:

hits	output
none hit	empty json array []
one item hit	one item json array ["itemValue"]
multiple hits	array of values ["itemA", "ItemB"....]

Code DmnActivityBehavior

        } else {
            boolean multipleResults = decisionExecutionAuditContainer.isMultipleResults() && processEngineConfiguration.isAlwaysUseArraysForDmnMultiHitPolicies();

[martin-grofcik]

@tijsrademakers, @jbarrez : What is your opinion? I can contribute with fix.