Sometimes a transformer can't return a subject. In this case transform() will return null
instead of throwing an exception. This means an automation can run until it comes to a
step where the subject would be needed. There it would fail.
[PREMIUM-248]
Given the trigger provides two subjects but only one subject was given via parameter and no
transformer was able to generate the second from the first, this method runs into an
infinite loop. This commit prevents this infinite loop.
[MAILPOET-4935]
All subjects are created and persisted during AutomationRun creation. Therefore, the required
subject must exist when searching for it in the StepHandler. This commit reverses the last
changes and updates the test.
[MAILPOET-4935]
