`No spec provided` message flashes while remote API definition loads

[shockey]

Q	A
Bug or feature request?	Bug
Which Swagger/OpenAPI version?	Any
Which Swagger-UI version?	3.4.4
How did you install Swagger-UI?	n/a
Which browser & version?	Chrome 62/Safari 11
Which operating system?	macOS High Sierra

Expected Behavior

No spec provided should only display if Swagger-UI doesn't expect to have an API definition in the future, barring external intervention.

Current Behavior

No spec provided displays while an API definition provided as a url config option is loaded.

Also, it appears as unformatted text in the top left corner of the container, which isn't great.

Possible Solution

• Don't display the message when a url is provided
• Don't display the message while a remote definition fetch is in progress
• Display a Loading message instead

Context

Born out of a discussion with @heldersepu in #3896.

[shockey]

Fixed by #4013 🎉