Un-collapse nothings in gradient

[ToucheSir]

Companion to FluxML/ZygoteRules.jl#26. After some discussion in #1466 and elsewhere, we decided preserving the Tuple return was the way to go.

PR Checklist

• Tests are added
• Documentation, if applicable

[ToucheSir]

Well, I didn't expect downstream tests to fare so well. Also only minimal changes to Zygote's own test suite required.

Rough next steps:

1. Cut a ZygoteRules release with Collapse tuples of nothing ZygoteRules.jl#26. I'm on the fence about whether to mark it as breaking right now, thoughts and opinions very much welcome.
2. Update this PR to use the new ZygoteRules and tweak doc(string)s to mention this guarantee about gradients return type.
3. Merge this and start working on newly unblocked PRs like Remove redundant sum() rules #1453 and Allow ChainRules zero types internally #1389.

[ToucheSir]

Docs are up so this should be ready. Amazingly, we never had a docstring for pullback despite referencing it in other parts of the docs!