Collapse zeros for `Tuple` & `Ref` tangents

[mcabbott]

This is one way not to make a Tangent with only zero types in it:

julia> ProjectTo(Ref(1))(Ref(1))  # ok
Tangent{Base.RefValue{Int64}}(x = 1.0,)

julia> ProjectTo(Ref(1))(Ref(NoTangent()))  # could collapse to NoTangent()
Tangent{Base.RefValue{Int64}}(x = NoTangent(),)

You could go further, and make Tangent{Foo}() and Tangent{Foo}(x = NoTangent()) all return NoTangent. But perhaps it's odd to make a a type constructor not return that type... this PR does not do that.

[codecov-commenter]

Codecov Report

Merging #565 (971eba9) into main (f2e3ac5) will decrease coverage by 0.09%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
- Coverage   93.14%   93.04%   -0.10%     
==========================================
  Files          15       14       -1     
  Lines         875      877       +2     
==========================================
+ Hits          815      816       +1     
- Misses         60       61       +1     

Impacted Files	Coverage Δ
src/projection.jl	96.98% <88.88%> (-0.33%)	:arrow_down:
src/rule_definition_tools.jl	96.81% <0.00%> (-0.05%)	:arrow_down:
src/ChainRulesCore.jl

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[mcabbott]

Xref discussion at #581, I guess that it would be slightly better for this to return a ZeroTangent than a NoTangent.

[mcabbott]

The stronger way to abstract this would just be to make the Tangent constructor return a Zero. I don't quite see why an all-Zero Tangent is ever better.