add workaround for non-existent directories not being reported in GL
add workaround for non-existent directories not being reported in GL

 * GitLab returns a 200 OK when queried for a directory that does not
   exist.  We can work around this by checking to see if the directory
   contains anything; empty directories shouldn't exist in git.

TomBaxter/waterbutler,RCOSDP/waterbutler,rdhyee/waterbutler,felliott/waterbutler,CenterForOpenScience/waterbutler