Definition XML import can get stuck on deleted definition

Problem:
With OPT-3634 situations like the decision data import getting stuck because of deleted definitions got prevented, however there is still an edge-case where the import can get stuck on the two-phase definition import:
1. Import of definition by deployment date
2. Import of definition XML for all definitions that don't have a XML stored in Optimize

If between 1 and 2 the definition got deleted the XML import get's stuck as trying to fetch the XML for a deleted definition throws a 400 bad request on the engine-rest API.

Steps to reproduce:
1 Deploy a definition run the Optimize import and kill the application 3-5s after start
2 check that the definition is present in the definition index but lacks the XML (alternatively update the definition via the elastic API to clear the bpmn20Xml field)

*Actual result : *
3 Start Optimize, the XML import should start throwing an exception on fetching the XML and is stuck from there on

*Expected result: *
The XML import could log a warn that the definition it's trying to fetch got deleted and just skip it and continue.

Hint:
Another kind of issue that can occur is that the XML is actually invalid and can't be parsed (e.g. due to breaking changes on xml validation). We could apply the same logic and just skip these XMLs and make this transparent via a log to not block the import in such a case as well, see SUPPORT-10797.

Lets also make sure to fix the related IT (see comment)

Testing Notes:

• Follow the steps to reproduce above
• Check the Optimize logs, the process and decision definition logs should be working as expected
• Check ES, the deleted definitions should be marked as deleted
• In the UI, the deleted definitions should not be selectable anywhere