Fix bug with imported while loops with C API enabled.

Specifically, make control_flow_util.GetOutputContext robust to
imported exit nodes (which don't have control flow contexts). This was
a bug prior to the C API being enabled in that imported exit nodes
would not have contexts, but it happened to not be exposed. Note that
importing a metagraph will add the contexts back after doing the
initial import, but there's still a window where no contexts are
assigned.

PiperOrigin-RevId: 180730785