Handle empty parameter items with missing type

[kylef]

Spotted parameters in the wild causing the parser to raise an exception due to type being undefined:

        - name: xxx
          in: query
          required: false
          type: array
          items: {}
          collectionFormat: multi

   1) Parameter to Member converter can convert a parameter to a member with array empty items

       Type is not a constructor

       at Parser.convertParameterToElement (src/parser.js:1284:19)

       1284 |     let element = new Type();

       at Parser.<anonymous> (src/parser.js:1334:33)

       1333 |       this.withPath('items', () => {
       1334 |         element.content = [this.convertParameterToElement(parameter.items, true)];
       1335 |       });