Process start, potentially migration: Signal scope gets ignored starting with the 2nd process

**Describe the bug**
If I start a process that has a Signal event definition and a Signal Start Event, starting it will work normally the first time.
The second time, the resulting event subscription's scope definition will always be "null", because the event definition's signal ref gets overwritten the first time.

Potentially, this may also affect process instance migrations (there are 2 places in the code where the behavior is like this).

**Expected behavior**
I expect the same behavior each time - the signalref stays as-is, the event subscription always gets created with the right scope.

**Code**
I prepared a Pull Request here: https://github.com/flowable/flowable-engine/pull/3920

**I would love to add regression tests as well, but I can't seem to find a good place.**
**Please point me to a "good" place where I can add a test method as a regression test.**

**Additional context**
All that should matter is the flowable version.
I tried with 6.8.1 and 7.0.1, the bug is in both and seems to go back very far.

**Workaround**
Without a fix, the bug does not occur if Signal ID and name must are the same, e.g.
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Esamina la pull request 3920 e riproduci due volte il flusso di avvio del processo: con una Signal event definition e con un Signal Start Event. Verifica che la sottoscrizione all’evento mantenga il signal scope a ogni avvio, quindi aggiungi un test di regressione nell’area di test di workflow-engine appropriata identificata durante il tracciamento di quel punto di ingresso.

Scritto dal modello di indicizzazione a partire dal testo della issue.