Bug: Template builder fails when loading data without outliner representation

[ClementHector]

Current Behavior:

If a template contains a placeholder without outliner representation it will produce this warning:

# # *** WRN: >>> { <openpype.hosts.maya.api.workfile_template_builder.MayaTemplateBuilder object at 0x7f152d504190> }: [ Failed to process placeholder |root|Production_render_template_23_5 with plugin MayaPlaceholderLoadPlugin ] 
# ==============================
# No reference node found in container
# ==============================
# Traceback (most recent call last):
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/pipeline/workfile/workfile_template_builder.py", line 727, in populate_scene_placeholders
#     placeholder_plugin.populate_placeholder(placeholder)
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/hosts/maya/api/workfile_template_builder.py", line 237, in populate_placeholder
#     self.populate_load_placeholder(placeholder)
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/pipeline/workfile/workfile_template_builder.py", line 1634, in populate_load_placeholder
#     self.load_succeed(placeholder, container)
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/hosts/maya/api/workfile_template_builder.py", line 261, in load_succeed
#     self._parent_in_hierarchy(placeholder, container)
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/hosts/maya/api/workfile_template_builder.py", line 278, in _parent_in_hierarchy
#     ref_node = get_reference_node(roots)
#   File "/users_roaming/chector/Documents/src/OpenPype/openpype/hosts/maya/api/lib.py", line 4023, in get_reference_node
#     assert references, "No reference node found in container"

Steps To Reproduce:

1. Add a placeholder without ouliner representation in your template (like maya render settings)
2. Build workfile from template
3. See this error in the script editor

[ClementHector]

PR => https://github.com/ynput/OpenPype/pull/5222