Generator fails to build models for schemas of type `object` with no/empty `properties`

[micolous]

The generator fails to build code or generates invalid code for a schemas that describe an empty object.

Build failure with no properties:

python-client-generator fails to build a model for an object with no properties key, eg:

{
 "components": {
  "schemas": {
   "EmptyObject": {
    "type": "object"
   }
  }
 }
}

Generating a client when properties is missing makes the generator raise KeyError('properties') at:

python-client-generator/python_client_generator/generate_models.py

Lines 99 to 102 in 05b2c51

	else:
	# Must have properties
	for p_schema in model["properties"].values():
	refs += _get_schema_references(p_schema)

Contrary to the code comment, in the generator, it is actually valid for an object to have no properties field. Per JSON Schema Core specification:

The value of "properties" MUST be an object. Each value of this object MUST be a valid JSON Schema.

...

Omitting this keyword has the same assertion behavior as an empty object.

Invalid code with empty properties

For an object with an empty properties key, eg:

{
 "components": {
  "schemas": {
   "EmptyObject": {
    "type": "object",
    "properties": {}
   }
  }
 }
}

The generator builds invalid Python code:

class EmptyObject(BaseModel):

There should be a pass in there when there are no fields.

Testing the fix

It looks like FastAPI always includes a properties key for empty objects (so only hits the second issue), whereas protoc-gen-openapiv2 (both before and after converting OAS v2 to OAS v3) does not (so hits both issues).

I've modified both the Pet Store samples to trigger both cases in #26.