parseRequest middleware does not allow empty array parameters

[JamesMGreene]

The parseRequest middleware does not allow empty arrays for specified parameters (e.g. /foo?username=), even when such a parameter's schema includes required: false and/or minItems: 0.

Error: The "username" query parameter is invalid ("")
JSON Schema validation error.
Invalid type: string (expected array)
    at ono (c:\git\Observer\Apps\ObsNode\node_modules\ono\lib\index.js:62:17)
    at parseParameter (c:\git\Observer\Apps\ObsNode\node_modules\swagger-express-middleware\lib\param-parser.js:148:11)
    at c:\git\Observer\Apps\ObsNode\node_modules\swagger-express-middleware\lib\param-parser.js:35:35
    at Array.forEach (native)
    at parseSimpleParams (c:\git\Observer\Apps\ObsNode\node_modules\swagger-express-middleware\lib\param-parser.js:30:12)
    at ...

However, as I understand it from OAI/OpenAPI-Specification#305 and APIDevTools/swagger-parser#6, this should result in [].

I can temporarily workaround the problem by adding default: [] to the schema but that also sets the parameter to [] even if it was not specified at all (i.e. for both /foo and /foo?username=).

[JamesMGreene]

Proposed PR #45.