Should JSON null be preserved? - json

Question

I tried to convert the JSON data
{
"a": {
"b": null
}
}
to XML using an online converter. The response was
<a>
<b />
</a>
Converting this back to JSON using the same converter gave me
{
"a": {
}
}
This made me wonder – if you're explicitly given a null value, are you required to preserve it when dealing with JSON? I'm fairly sure that the XML <a><b /></a> is not equivalent to <a></a>, and especially not <a /> (which happens to be what I get when I continue with the same exercise).
In other words, if I'm handed JSON of unknown origin and am supposed to hand it over to an unknown recepient, am I required to preserve the nulls or can I safely remove them? Conversely, can I rely on my nulls to end up in the same way I outputted them when delivered by third-party software?
Here's a similar question: Should JSON include null values – However, the question there is whether the code should output nulls if you define the format yourself, not what you should do if you don't know anything about the original format.
EDIT – Clarification: The way I asked the question was bad and apparently caused confusion. To rephrase it: I do understand that XML and JSON are different formats and are able to carry different kinds of (meta)data. I do know that null is a valid value, as defined by RFC4627. I do understand that there are different ways to convert between XML and JSON since the formats don't have a 1-to-1 relationship. I do understand that the converter I found might be buggy. However, the fact that the same converter didn't provide the same conversion in both directions (no information was lost when converting from "b": null to <b /> and a similar translation in the opposite direction would have been possible) made me wonder something that I couldn't find an answer to despite attempts:
Is it legal, according to the JSON standard, to treat {"a":{"b":null}} and {"a":{}} as one and the same object when transferring them on behalf of other software?
Note that I'm here assuming that it's legal to add or remove whitespace as I see fit (e.g. pretty-printing, which is okay according to RFC4627), and even to rearrange the name/value pairs in a collection (again according to RFC4627). I simply don't know if a null must be preserved in the same way as significant data, or can be dropped in the same way as insignificant whitespace.

Answer 1

Yes, null is a separate value in JSON, and is distinct from not having an attribute, obviously. Also, you can see this question about nulls in XML. The thing to conclude here isn't that there is something wrong with JSON or XML, but simply that the tools you use aren't coded to handle these cases.

Answer 2

One of the problems in converting JSON to XML is that if you try and make the conversion lossless, you end up with somewhat "unnatural" XML, whereas if you try to create the most natural XML representation, it ends up losing information. That's why there are lots of different converters that all do it in slightly different ways. Choose the one that meets your requirements.