[JENKINS-72886] fix command & args are lost when combining container templates

[moleus]

The issue is described in detail here - https://issues.jenkins.io/browse/JENKINS-72886

This PR adds checks for empty Lists because args and command fields are initialized as empty lists by default, and they can't be null. Before this change args and command from parent template were overridden with empty values, because null check always returned false.

After this change args and command fields will be preserved from parent template if child don't provide any. This is the expected behavior.

Testing done

I've added simple unit test which failed before this change and now passes successfully. It check that fields from parent container template are present if child template doesn't contain these fields

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[Vlatombe]

Thank you!