fix one/anyOf for objects without properties

[fliem]

entity of type "object" that includes "oneOf" " or "anyOf" but no "properties" falls through the cracks (see below).

This PR fixes this.

Traceback (most recent call last):
  File "jsf/src/jsf/schema_types/object.py", line 40, in generate
    return super().generate(context)
  File "jsf/src/jsf/schema_types/base.py", line 49, in generate
    raise ProviderNotSetException()
jsf.schema_types.base.ProviderNotSetException

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "jsf/src/jsf/schema_types/object.py", line 40, in generate
    return super().generate(context)
  File "jsf/src/jsf/schema_types/base.py", line 49, in generate
    raise ProviderNotSetException()
jsf.schema_types.base.ProviderNotSetException

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "jsf/src/jsf/schema_types/object.py", line 40, in generate
    return super().generate(context)
  File "jsf/src/jsf/schema_types/base.py", line 49, in generate
    raise ProviderNotSetException()
jsf.schema_types.base.ProviderNotSetException

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "mockdatageneration/prototype/test.py", line 19, in <module>
    data = faker.generate()
  File "jsf/src/jsf/parser.py", line 143, in generate
    return self.root.generate(context=self.context)
  File "jsf/src/jsf/schema_types/object.py", line 42, in generate
    return {o.name: o.generate(context) for o in self.properties if self.should_keep(o.name)}
  File "jsf/src/jsf/schema_types/object.py", line 42, in <dictcomp>
    return {o.name: o.generate(context) for o in self.properties if self.should_keep(o.name)}
  File "jsf/src/jsf/schema_types/object.py", line 42, in generate
    return {o.name: o.generate(context) for o in self.properties if self.should_keep(o.name)}
  File "jsf/src/jsf/schema_types/object.py", line 42, in <dictcomp>
    return {o.name: o.generate(context) for o in self.properties if self.should_keep(o.name)}
  File "jsf/src/jsf/schema_types/object.py", line 42, in generate
    return {o.name: o.generate(context) for o in self.properties if self.should_keep(o.name)}
TypeError: 'NoneType' object is not iterable

[ghandic]

Thanks for the PR! Great catch