Check whether default_profile has been set
Check whether default_profile has been set

Add a probaly forgotten check of default_profile emptiness. As
stated in comment in kobo/cli.py, line 288 (PR #148):

  this block should only be evaluated if default_profile has
  been set at instantiation

_load_profile() should be executed only if default_profile is
set/nonempty, otherwise there is a danger of passing empty
strings and getting paths like /etc/.conf (see issue #151).

release-engineering/kobo,release-engineering/kobo,release-engineering/kobo,release-engineering/kobo