Even though the code as-is *should* be safe, given that we're using an Object with a `null` prototype, it cannot hurt to change this to a Map to prevent any issues (since we're parsing unknown and potentially unsafe data).
Overall I also think that these changes improve the `parseQueryString` call-sites, since we now have a proper way of checking for the existence of a particular key (and don't have to use `in` which stringifies the keys in the Object).
This patch also changes the default, when no `value` exists, from `null` to an empty string since the use of `decodeURIComponent` currently can modify the value in a somewhat surprising way (at least to me).
Note how `decodeURIComponent(null) === "null"` which is unlikely to be what you actually want, whereas `decodeURIComponent("") === ""` which seems much more helpful.
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