#1305 fixing parsing of empty securityRequirement

[wsalembi]

Because in the v1 library operation.getSecurity() doesn't return a list of SecurityRequirements cfr the top-level security field in the swagger file, I opted for adding an empty Map to the list to represent the none security requirement.

We need swagger-parser to retain the unsecured security requirement on an operation level. Now it is stripped

[gracekarina]

thanks @wsalembi