lone custom outputs ignored

[hjoliver]

This validates OK, and :x is ignored in the completion expression (same for a:x?):

[scheduling]
   [[graph]]
        R1 = "a:x"
[runtime]
   [[a]]  # no outputs defined

But triggering another task off the output results in a warning:

[scheduling]
   [[graph]]
        R1 = "a:x => b"
[runtime]
   [[a]]  # no outputs defined
   [[b]]

WorkflowConfigError: Undefined custom output: a:x