Resolve NXCALS-6818 Make system optional if specify id

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With this change, it will be possible to specify the id of an entity or variable without specifying the system.

DataQuery.builder(spark).variables().idEq(1L).timeWindow("2047-01-01 00:00:00", "2047-01-02 00:00:00").build()

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@acc-logging-team - please have a look for approval.

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Closes NXCALS-6818