Make `arch` and `platform` inside index.json optional

[0xbe7a]

When a noarch package is built, conda-build currently sets the arch and platform fields to null. rattler-build does not even include them when a noarch package is built, as they are optional anyway (see https://docs.conda.io/projects/conda-build/en/stable/resources/package-spec.html#repo-si). This PR sets arch and platform to null instead of panicking if the fields are missing.

[wolfv]

Awesome! Did you try and upload a package like this? I am just wondering if some other parts of the Anaconda infrastructure might have problems with these fields missing. We can also change the serde configuration in rattler to emit these fields with null values ...

[0xbe7a]

I tried uploading a package created by rattler-build to anaconda, and apart from this small problem, everything worked perfectly. My initial fix was to include the null in the index.json, but it seems that the better solution is to make the anaconda client follow the specification here, since the fields are optional anyway.

[dholth]

Reminds me of https://github.com/conda/conda-index/issues/132

Many packages do and many do not include redundant information in the index fragment for repodata.json

[pavelzw]

@vshevchenko-anaconda could you take a look at this PR? Would be great to get this merged.

[wolfv]

yaaay!