Add support for parameters.allowEmptyValue

[ahenket]

Reading up in the openapi specs I also came across allowEmptyValue which by default is false. This is not enforced by roaster at all yet and doing so would be a major breaking change for people who do not specify allowEmptyValue yet.

But it is useful to specify not to use a parameter OR to have a value for it.

How this could work in a non-breaking way in parameters:retrieve#1. Doesn't take this into account yet.

If style is used, and if behavior is n/a (cannot be serialized), the value of allowEmptyValue SHALL be ignored.

declare %private function parameters:retrieve ($parameter as map(*)) as map(*)? {
    if (parameters:is-path-parameter($parameter))
    then ()
    else
        let $name := $parameter?name
        let $default := parameters:get-parameter-default-value($parameter?schema)
        let $needsValue := exists($parameter?allowEmptyValue[. = false()])
        let $values := 
            switch ($parameter?in)
            (: TODO case "body" :)
            case "header" return
                head((request:get-header($name), $default))
            case "cookie" return
                head((request:get-cookie-value($name), $default))
            default return
                request:get-parameter($name, $default)

        return
            if ($parameter?required and empty($values)) then
                error($errors:REQUIRED_PARAM, "Parameter " || $name || " is required")
            else
            if ($needsValue and exists($values[string-length() = 0])) then
                error($errors:REQUIRED_PARAM, "Parameter " || $name || " needs a value when present")
            else
                map { $name : parameters:cast($values, $parameter) }
};