move (( empty )) into merge time

[thomasmitchell]

The following breaks, but it might make more sense for inject to just inject an empty map. Instead, it tries to inject the string (( empty map )), and then it dies. (( empty ... )) should probably be moved into a phase where (( inject ... )) can see it as a dependency and hold off until the map becomes an actual empty map.

---
meta:
  foo:
    beep: boop

---
stuff:
  name: geoff
  .: (( inject meta.foo ))

---
meta:
  foo: (( empty map ))

[geofffranks]

closing as stale