Show a helpful message by default (instead of the Petstore)

[shockey]

Content & configuration

An unconfigured Swagger UI instance, i.e.:

SwaggerUI({})

Is your feature request related to a problem?

As a user trying to get started with Swagger UI, it's quite possible to hit a point where I get Swagger UI working but haven't successfully pointed it at my definition.

Consequentially, I'm going to see the Petstore served by default, which is a nice demo but doesn't get me any closer to showing my content in Swagger UI.

I've probably already seen and played around with the Petstore while shopping around for documentation tools, so showing it to me again doesn't provide me with any value.

Describe the solution you'd like

A helpful message similar, something like:

No API definition was provided to Swagger UI.

You should provide an API definition as a URL or as a string in 
Swagger UI's configuration.

[Click here] for documentation on how to get started with Swagger UI.

We do something similar today when a user feeds us an indeterminate-version document (i.e., one that is valid JSON/YAML but has no valid swagger/openapi field value):