[build-tools] Default missing outputs to empty string

[sjchmiela]

Why

https://exponent-internal.slack.com/archives/C06EFBQK3B7/p1737419576613249

How

When we're collecting job outputs from steps, we treat values as strings which may include template expressions. So:

outputs:
  foo: bar # sets foo="bar"
  build_id: ${{ steps.find_build.outputs.build_id }} # acts how `${steps.find_build.outputs.build_id}` in JS would act
  key: ios-${{ ... }} # concatenates "ios-" and whatever template expression evaluates to
  counter: ${{ 2 + 3 }} # sets counter="5"

I see a way of reasoning about outputs that they are never expected to be used as static values, i.e. foo: bar and key: ios-${{ ... }} are never what the user will want to use and instead we should always use value as an expression. However,

outputs:
  foo: "\"bar\""
  build_id: steps.find_build.outputs.build_id
  key: "\"ios-\" + ..."
  counter: 2 + 3

looks strange to me.

We could also not stringify results and do what James suggests, let undefined be undefined. However, that would also mean 2 is 2, not "2". And I think it would be better for users that do want this level of type-correctness to explicitly do counter: ${{ toJSON(2) }} and then fromJSON(needs.setup.outputs.counter).

Test Plan

Added test.

[linear]

ENG-14785 Make empty output value `''`, not `undefined`