Maya: Account for no nodes in container

[tokejepsen]

Changelog Description

When builder a template with renderSetup product, this error would occur since renderSetup product now does not return any roots after #310;

# Error: AssertionError: file C:\Users\tokejepsen\ayon-core\client\ayon_core\hosts\maya\api\testing\tests.py line 251: Failed to process placeholder "|Context_rendersetup_folder_template_json" with plugin "MayaPlaceholderLoadPlugin"
# Traceback (most recent call last):
#   File "C:\Users\tokejepsen\ayon-core\client\ayon_core\pipeline\workfile\workfile_template_builder.py", line 717, in populate_scene_placeholders
#     placeholder_plugin.populate_placeholder(placeholder)
#   File "C:\Users\tokejepsen\ayon-core\client\ayon_core\hosts\maya\api\workfile_template_builder.py", line 246, in populate_placeholder
#     self.populate_load_placeholder(placeholder)
#   File "C:\Users\tokejepsen\ayon-core\client\ayon_core\pipeline\workfile\workfile_template_builder.py", line 1639, in populate_load_placeholder
#     self.load_succeed(placeholder, container)
#   File "C:\Users\tokejepsen\ayon-core\client\ayon_core\hosts\maya\api\workfile_template_builder.py", line 280, in load_succeed
#     self._parent_in_hierarchy(placeholder, container)
#   File "C:\Users\tokejepsen\ayon-core\client\ayon_core\hosts\maya\api\workfile_template_builder.py", line 302, in _parent_in_hierarchy
#     for root in roots:
# TypeError: 'NoneType' object is not iterable

Testing notes:

1. Setup placeholder with renderSetup product in Maya.
2. Build the templated workfile.

[BigRoy]

Didn't test - but code change makes sense. maya.cmds.sets does indeed return None when empty.

[moonyuet]

Tested successfully in maya.