fix: runStage null to non-null or cancel via exception

[jbee]

Summary

The runStage method that would run a Callable could return null either from that Callable or from the errorValue given.
This potential NPE was not handled in many places. Often enough it also wasn't an issue as the usage of the returned value was below the next startingStage which would fail if the null was due to an error so the code would never proceed to use the variable.

With the JobProgress not having nullness annotations the nullness analysis would warn about potential NPEs from the return values of runStage. To transition the value into a known non-null while keeping the error behaviour of throwing a cancellation exception on next use of progress after a failed stage a new method was added to confirm post run assumptions:

The nonNullStagePostCondition method takes a potential null value and simply returns it when non-null or throws an exception when it is null.

I checked the callers of the runStage method and added nonNullStagePostCondition where the nullness analysis indicated potential NPEs

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud