building an oas3BaseUrl fails if servers == []

[dtkav]

environment:

swagger-js version: v3.8.0

test case:

const spec = {
  openapi: '3.0.0',
  servers: []
}

const res = baseUrl({
  spec,  
  server: 'http://some-invalid-server.com/',
  contextUrl: 'http://google.com/'
})

expect(res).toEqual('http://google.com')

error:

  1) buildRequest - OpenAPI Specification 3.0 baseUrl should fall back to contextUrls if servers list is empty:
     TypeError: Cannot read property 'url' of undefined
      at oas3BaseUrl (src/execute/index.js:284:25)
      at baseUrl (src/execute/index.js:261:23)
      at Context.<anonymous> (test/oas3/execute/main.js:799:19)

potential fix:

dtkav@ca2a112

how I found the bug:

swagger-api/swagger-ui#4505