[FIXED JENKINS-20559] - ProcStarter: Check for null before cloning of environment variables list

Resolves https://issues.jenkins-ci.org/browse/JENKINS-20559
BTW, I also need to add some tests to "Test Harness" in order to improve coverage of nested operations.
These test require a fix for https://issues.jenkins-ci.org/browse/JENKINS-19454



Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>