Fix toQueryString function in UrlUtil for null values

[chrismayer]

This PR fixes the toQueryString function in the UrlUtil in case a null value is passed in for a parameter. Main reason is that typeof null resolves to 'object' (thanks JavaScript 🙄 ), which leads to a recursion with null instead of an object.

[fschmenger]

Hi Christian, looks good to me. One question here: Is it required to encode params as null or should they just be removed from the query string?
If the answer is 'yes' then feel free to merge.

[chrismayer]

Thanks for your review @fschmenger. I don't think it is required to encode URL params as null and maybe a bad idea if I think twice. There might be typing problems in some API implementations receiving foo=null, since null is then a string and might not match a defined data type.

So I decided to rework this a bit and remove null values from the URL query string. Would you please give this another quick check, @fschmenger ? If you are OK with the code please hit the Approve button, otherwise the PR is not mergable... Thanks in advance.

[fschmenger]

Ok, looks good.

[chrismayer]

Thank you @fschmenger 🙏 Going to merge now...