create-pull-request: handle empty ODIR

In some situations you might end-up with an empty ODIR (pull-xx/). The
most common reason is that you have applied your patches on 'master'
branch (or you are by mistake standing on the 'master' branch),
this will result in the default behavior that 'git format-patch'
will try to diff master..master.

Solve this by aborting the script with a proper error code and message
if ODIR is empty after the 'git format-patch' call (that is expected
to generate the cover-letter and patches).

Signed-off-by: Petter Mabäcker <petter@technux.se>
Signed-off-by: Ross Burton <ross.burton@intel.com>