function Component(props) {
  let x = [];
  if (ENABLE_FEATURE) {
    x.push(42);
    return x;
  } else {
    console.log('fallthrough');
  }
  return makeArray(props.a);
}
This fixture tests what happens when a reactive has no declarations (other than an early return),
no reassignments, and no dependencies. In this case the only thing we can use to decide if we
should take the if or else branch is the early return declaration. But if that uses the same
sentinel as the memo cache sentinel, then if the previous execution did not early return it will
look like we didn't execute the memo block yet, and we'll needlessly re-execute instead of skipping
to the else branch.

We have to use a distinct sentinel for the early return value.

Here the fixture will always take the "else" branch and never early return. Logging (not included)
confirms that the scope for `x` only executes once, on the first render of the component.