Fix call to non-existing newWorkCommands() in NuanceItemType

Summary:
rPe1b8532e2478395bd1f36baecb998adeef5bea3d introduced an implementation of newWorkCommands() in the child class NuanceFormItemType which is called in parent NuanceItemType::buildWorkCommands() without being defined in that parent class.
Thus define newWorkCommands() as an abstract class in parent class NuanceItemType and add an implementation returning null in the other child class NuanceGitHubEventItemType to make static code analyzers happy.

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25655