If an empty group is the default group, then treat it as empty on purpose

[paulhovey]

This is continuing the discussion from #480 (comment) . Essentially, I was looking for a way to treat gitman slightly less of a default package manager, and a little more of a selective "please clone these repos recursively but not this one". Groups were selected which was a good idea in theory, but for a subrepo with a gitman.yml that we only wanted the top level of this solution fell short because gitman was treating an empty default group (even one defined, but defined with no members) as being empty, and cloning all repos/groups.

I explicitly left the current case of default_group: '' or default_group being undefined as defaulting to the current behavior of cloning everything, you have to explicitly define an empty group to get the behavior of not cloning any repos:

default_group:  empty
groups:
  - name: empty
    members:  null