[oad] Unhandled Exception: System.ArgumentNullException

[vishrutshah]

Swagger: specification/visualstudio/resource-manager/Microsoft.VisualStudio/2014-04-01-preview/Csm.json

CI: https://travis-ci.org/Azure/azure-rest-api-specs/jobs/259997569#L697

Exception:

Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: source
   at System.Linq.Enumerable.Select[TSource,TResult](IEnumerable`1 source, Func`2 selector)
   at AutoRest.Swagger.Model.Operation.CheckParameters(ComparisonContext context, Operation priorOperation)
   at AutoRest.Swagger.Model.Operation.Compare(ComparisonContext context, SwaggerBase previous)
   at AutoRest.Swagger.Model.ServiceDefinition.Compare(ComparisonContext context, SwaggerBase previous)
   at OpenApiDiff.Program.Main(String[] args)
Aborted (core dumped)
    at ChildProcess.exithandler (child_process.js:270:12)
    at emitTwo (events.js:125:13)
    at ChildProcess.emit (events.js:213:7)
    at maybeClose (internal/child_process.js:921:16)
    at Socket.stream.socket.on (internal/child_process.js:348:11)
    at emitOne (events.js:115:13)
    at Socket.emit (events.js:210:7)
    at Pipe._handle.close [as _onclose] (net.js:549:12)
  killed: false,
  code: 134,
  signal: null,
  cmd: '/home/travis/.autorest/frameworks/dotnet /home/travis/build/Azure/azure-rest-api-specs/node_modules/oad/lib/dlls/OpenApiDiff.dll -o /tmp/old.json -n /tmp/new.json -JsonValidationMessages' }