Filter fields with null values in exported dashboard model

[sdboyer]

One background goal with schematization is that we disallow any field, in any schematized object, from containing an explicit null value. Much ink could be spilled over this, but it is essentially because it reduces ambiguity: by banning nulls universally, we make it so that no one authoring a schema ever has to answer the question, "if a field exists with value null, is that meaningfully different from that field being absent?"

#41385 helped a lot with this by purging nulls from dashboard JSON that's sent for save. But we're still producing nulls on export, which is why PRs like #54124 end up happening - to fix something that was exported (and avoided the test that should have caught it that i only just fixed in #53797 😱).

We need to go the additional step of also stripping nulls from exported dashboard JSON.

cc @joshhunt @ryantxu

[joshhunt]

We use Infinity in the dashboard model (as a sentinel value). When browsers stringify Infinity, it converts it to null. sortedDeepCloneWithoutNulls doesn't handle this.

[sdboyer]

This feels like one case where it'd be OK to break the "no nulls in JSON" rule. That's simply because, if browsers stringify Infinity as null, then that's a) a meaningful null, because it is being used to represent a value that is not in the set of values directly representable with number, and b) doesn't require any special handling in our serialization algorithm.

That said, i am concerned about how we would account for JS having both negative and positive infinity - null can only be assumed to correspond to one of those.

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