Fix to remove all empty categories in action menu

[davidlatwe]

Problem

Plugins which using multiple Category or Separator to organize Action may produce an action menu that left some Category or Separator uncleaned in GUI.

As you can see, in those two action menus, there are an empty Category C and a Separator with no actions to separate.

Fix

I like to use Category, and with this PR, redundant Category and Separator would be taken care of.

🍺 🍺

Reproducible Code

```python import pyblish.api class CollectInstances(pyblish.api.ContextPlugin): order = pyblish.api.CollectorOrder def process(self, context): context.create_instance(name="Dummy") class ActionToAll(pyblish.api.Action): on = "all" label = "Action to All" class ActionOnFailed(pyblish.api.Action): on = "failed" label = "Action On Failed" class ValidateFailed(pyblish.api.InstancePlugin): order = pyblish.api.ValidatorOrder label = "Many Categories" actions = [ pyblish.api.Category("A"), ActionToAll, pyblish.api.Category("B"), ActionOnFailed, pyblish.api.Category("C"), ActionOnFailed, ] def process(self, instance): assert True is False class ValidatePass(pyblish.api.InstancePlugin): order = pyblish.api.ValidatorOrder label = "With Separator" actions = [ pyblish.api.Category("A"), ActionToAll, pyblish.api.Separator, ActionOnFailed, ] def process(self, instance): assert True pyblish.api.register_plugin(CollectInstances) pyblish.api.register_plugin(ValidateFailed) pyblish.api.register_plugin(ValidatePass) ```

[mottosso]

Nice one. :) Welcome to merge. 👍

[davidlatwe]

Merge !