Better code gen support for additionalProperties=true

[lyuyangh]

The generated class is empty (swagger_types is empty) if additionalProperties=true is used in openapi.yaml. It should at least list the properties that are defined. This issue applies to both flask-python and the ts client so it may be an issue related to the code gen tool (it may not be something related to the templates).

Not an immediate blocker but it'll be nice to fix this.

Related code is here:

explainaboard_web/backend/templates/util.mustache

Line 106 in 3ddab37

# If swagger_types is empty, klass has additionalProperties=true. data