no-op propertyNames causes invalid Python code to be generated

[henryiii]

This is supposed to be valid, I think (and it's in the new Pixi JSONSchema):

"propertyNames": {
  "description": "A valid task name."
}

But it causes fastjsonschema to emit an empty try block, which is invalid Python syntax. See henryiii/validate-pyproject-schema-store#224 and prefix-dev/pixi#1089.