Fix regression introduced in WorkflowRuntimeParameters#getParamValueWithUndefinedCheck() method

[marekhorst]

After introducing minor refactoring in WorkflowRuntimeParameters while developing #1398 one regression slipped in related to WorkflowRuntimeParameters#getParamValueWithUndefinedCheck() where istead of null a defaultValue should be returned.

While running WorkflowRuntimeParametersTest after refactoring was made all tests succeded which proved the test coverage was not sufficient and should be extended. Fortunately other SoftwareHeritageOriginsImporterTest test revealed the problem but it should be indicated closer to the root cause.