ADPT-164: Set default value for taskana.manual-priority

[ensarevlideveloper]

Checks if there are tasks (from previous versions) in the event_store and sets it to -1. Test creates task with manual_priority is null. Would normally throw NumberFormatException with fix, runs without exceptions.
Sonar-Link

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For the submitter:

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• I did not update the documentation
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Verified by the reviewer:

• Commit message format → ADPT-XXX: Your commit message.
• Submitter's update to documentation is sufficient
• SonarCloud analysis meets our standards
• Update of the current release notes reflects changes
• PR fulfills the ticket
• Edge cases and unwanted side effects are tested
• Readability