[build-tools] Default missing output values to empty string (#500)

# 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:
```yaml
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,
```yaml
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.

packages/build-tools/src/utils/__tests__/outputs.test.ts

@@ -132,6 +132,19 @@ describe(getJobOutputsFromSteps, () => {
       })
     ).toEqual({ fingerprint_hash: 'abc' });
   });
+
+  it('defaults missing values to empty string', () => {
+    expect(
+      getJobOutputsFromSteps({
+        jobOutputDefinitions: {
+          fingerprint_hash: '${{ steps.setup.outputs.fingerprint_hash }}',
+        },
+        interpolationContext: {
+          steps: { setup: { outputs: {} } },
+        },
+      })
+    ).toEqual({ fingerprint_hash: '' });
+  });
 });
 
 describe(uploadJobOutputsToWwwAsync, () => {

packages/build-tools/src/utils/outputs.ts

@@ -62,7 +62,7 @@ export function getJobOutputsFromSteps({
   const jobOutputs: Record<string, string | undefined> = {};
   for (const [outputKey, outputDefinition] of Object.entries(jobOutputDefinitions)) {
     const outputValue = outputDefinition.replace(/\$\{\{(.+?)\}\}/g, (_match, expression) => {
-      return `${jsepEval(expression, interpolationContext)}`;
+      return `${jsepEval(expression, interpolationContext) ?? ''}`;
     });
 
     jobOutputs[outputKey] = outputValue;