Update smtk core operation result attributes

Updates the smtk core operations that produce a resource to make the operation's resource assignment in the Result attribute optional. Currently, if one of these operation fails and the Result attribute is not updated, that Result attribute is invalid. This in turn triggers an exception when the Result attribute is traversed by the recently-added qtWorkletModel, which crashes modelbuilder.

fwiw (i) It might be prudent to put a try-catch block(s) in qtWorkletModel to avoid the crash; also (ii) We should probably check smtk operations ouutside of core for this same "invalid result attributes" potential.