Body/WebFormBody can't handle null bodies.

[brendo]

The current implementation of Body/WebFormBody's createFromArray method does not support the shortcut: !!null. Judging by the spec, this is legitimate:

/jobs:
  post:
    description: Create a Job
    body:
      text/xml: !!null
      application/json: !!null

At present, using RamlParser over that schema will result in:

PHP Catchable fatal error:  Argument 2 passed to Raml\Body::createFromArray() must be of the type array, string given, called in alecsammon/php-raml-parser/src/Method.php on line 143 and defined in alecsammon/php-raml-parser/src/Body.php on line 87

We currently call createFromArray from three locations, Method, Response and SecuritySchemeDescribedBy and unfortunately, we are inconsistent.

Response does not check the key for a valid media type, and it attempts to loosely handle a 'falsey' body by passing an empty array. Unfortunately !!null isn't going to trigger this logic, so it too will also fail.

I suggest we update all three calling locations to correctly handle !!null