Fix nesteddict2yaml not to fumble empty dict values

[sakurai-youhei]

This PR fixes the internal nesteddict2yaml() function to process empty dict values properly.

Without this PR, empty dict values are rendered to null so that openapi-spec-validator fails on the Swagger Spec file generated from app["SWAGGER_DEF_CONTENT"].

from aiohttp import web
import aiohttp_swagger

app = web.Application()
definitions = dict(
    test={
        "type": "object",
        "properties": {"prop": {"type": "object", "default": dict()}},
    }
)
aiohttp_swagger.setup_swagger(app, ui_version=3, definitions=definitions)

with open("spec.json", "w") as fp:
    print(app["SWAGGER_DEF_CONTENT"], file=fp)

{
  "openapi": "3.0.1",
  "info": {
    "title": "Swagger API",
    "description": "Swagger API definition",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "/"
    }
  ],
  "components": {
    "schemas": {
      "test": {
        "type": "object",
        "properties": {
          "prop": {
            "type": "object",
            "default": null
          }
        }
      }
    }
  },
  "paths": {}
}

spec.json: Validation Error: None for not nullable

Failed validating 'type' in schema:
    {'type': 'object', 'default': None}

On instance:
    None

Fixes: #106

P.S. Thank you for your great work on this project.