Make Fields interpret null values as empty strings

[lorban]

HttpRequest on the client used to interpret null parameter values as empty strings in Jetty 11, and some of the 12 code still contains such checks, e.g.:

    private String urlEncode(String value)
    {
        if (value == null)
            return "";

        return URLEncoder.encode(value, StandardCharsets.UTF_8);
    }

but the Fields class throws NPE when a null value is given.

This PR makes Fields consider a null value as empty strings.

Fixes #12018