Ensure error is raised when URI database name is parsed as 'None'.

You get a rather cryptic error when the `MONGO_URI` config value is missing a database path because `uri_parser.parse_uri(app.config[key'URI'])` returns `{…, "database": None, …}` and `PyMongo.init_app(…)` only checks for the non-existence of the `"database"` key.

This patch fixes that.