Make scheduled builds work for jobs with a Matrix Combinations Parameter

When a scheduled build with a Matrix Combinations Parameter is started, it is built with no combinations.

The reason is that the method createValue(StaplerRequest) in MatrixCombinationsParameterDefinition.java returns an instance of MatrixCombinationsParameterValue with an empty confs array and an empty values array for this case.
As a consequence combinationExists returns false for all combinations.

This quick fix causes combinationExists to always return true in case of an empty values array so that the Matrix Combinations Parameter can be used in jobs with a cron trigger.

Pull request for [JENKINS-21851]