Do not require "properties" to be specified by default for nodes

[yGuy]

Describe the bug For simple cases, it should not be required to provide "properties" for nodes.

To Reproduce Steps to reproduce the behavior:

1. Start with the introduction sample
2. Omit "properties" from one of the nodes:

   w.nodes = [
   {"id": 0},
   {"id": "one", "properties": {"firstName": "Bravo", "label": "Person B"}},
   {"id": 2.0, "properties": {"firstName": "Charlie", "label": "Person C", "has_hat": False}},
   {"id": True, "properties": {"firstName": "Delta", "label": "Person D", "likes_pizza": True}}
   ]

3. Show the widget using w.show()
4. Observe the error

   File /opt/conda/lib/python3.9/site-packages/yfiles_jupyter_graphs/widget.py:976, in GraphWidget.default_element_label_mapping(index, element)
   939 @staticmethod
   940 def default_element_label_mapping(index: int, element: TDict):
   941     """The default label mapping for graph elements.
   942 
   943     Element (dict) should have key properties which itself should be a dict.
   (...)
   974 
   975     """
   --> 976     properties = element['properties']
   977     return str(properties.get('label', properties.get('yf_label', index)))

Expected behavior This should be required by default. Some people have nodes without properties and it would still be useful.