Error raised when processing an object schema with no properties

[eslavich]

@PaulHuwe ran into an error when supplying DataModel with a schema containing an object with no properties:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/phuwe/src/roman/romancal/romancal/datamodels/reference_files/referencefile.py", line 10, in __init__
    super().__init__(init=init, **kwargs)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/model_base.py", line 165, in __init__
    self._schema = mschema.merge_property_trees(schema)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 202, in merge_property_trees
    walk_schema(schema, callback)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 149, in walk_schema
    recurse(schema, [], None, ctx)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 129, in recurse
    recurse(sub, path, c, ctx)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 137, in recurse
    recurse(val, path + [key], combiner, ctx)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 137, in recurse
    recurse(val, path + [key], combiner, ctx)
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 124, in recurse
    if callback(schema, path, combiner, ctx, recurse):
  File "/Users/phuwe/anaconda3/envs/roman-dev/lib/python3.9/site-packages/stdatamodels/schema.py", line 194, in callback
    del schema['properties']
KeyError: 'properties'

This works fine in ASDF so it should probably work here as well. The use case is a schema that makes optional properties described in the core schema required.