Handle null and undefined more similarly to query-string

[heygrady]

The way this library deals with some falsey values is different from the other competing libraries.

https://repl.it/@heygrady/querystring

Ideally:

• ?a would parse as { a: null } (like query-string does)
• { a: null } would stringify to a (like query-string does)
• { a: undefined } would stringify to an empty string (like all others do)
• FWIW, querystringify is the only one that stringifies null to "null" and undefined to "undefined"
• query-string has a unique handling of null that enables props to not have an equals sign (which is nice for aesthetics and distinct from empty string)
• The special cases of a=null and a=undefined will cause issues in other libraries

This library is nice because it's small. Good work!

	Size	Link
querystring	85B	https://bundlephobia.com/result?p=querystring@0.2.0
querystringify	514B	https://bundlephobia.com/result?p=querystringify@2.0.0
query-string	3kB	https://bundlephobia.com/result?p=query-string@6.2.0
qs	7.3kB	https://bundlephobia.com/result?p=qs@6.5.2

[3rd-Eden]

I agree, and the fact that it transforms null to an actuall string "null" is a bug. Fixed all of this in querystringify@2.1.0. Thanks for the issue report <3