Allow "processor" to be passed as scan options

[pauljohnston2009]

Fixes issue with parsing 'processor' config option, it would always fall through to new Pipeline($config['processors']) which would throw as its null

TypeError: OpenApi\Pipeline::__construct(): Argument #1 ($pipes) must be of type array, null given

current code worked when passing 'processors', but removed support for 'processor' option.

https://github.com/zircote/swagger-php/commit/2d983ce67b9eb7e18403ae7bc5e765f8ce7b8d56