[oozie] Handle non specified coordinator done-flag

The documentation on the Dataset creation says:

The done file for the data set. If the Done flag is not specified, then Oozie configures Hadoop to create a _SUCCESS file in the output directory. If Done flag is set to empty, then Coordinator looks for the existence of the directory itself.

This is from Oozie's documentation (as of OOZIE-1804):

• If the done-flag is omitted the coordinator will wait for the presence of a _SUCCESS file in the directory (Note: MapReduce jobs create this on successful completion automatically).
• If the done-flag is present but empty, then the existence of the directory itself indicates that the dataset is ready.
• If the done-flag is present but non-empty, Oozie will check for the presence of the named file within the directory, and will be considered ready (done) when the file exists.

Either way Hue currently always adds at least an empty done-flag. There's no way to leave it out entirely so the documentation should reflect that Hue only knows two states at the moment. Hue can't distinguish between empty and not-specified.