sendmail: handle properly non-koji-task builds
sendmail: handle properly non-koji-task builds

Avoid TypeErrors when build was triggered without task.

```
'Traceback (most recent call last):'
'  File "/usr/lib/python2.7/site-packages/atomic_reactor/plugins/exit_sendmail.py", line 386, in _get_logs_url'
"    url = '/'.join([pathinfo.work(), pathinfo.taskrelpath(self.koji_task_id)])"
'  File "/usr/lib/python2.7/site-packages/koji/__init__.py", line 2137, in taskrelpath'
'    return "tasks/%s/%s" % (task_id % 10000, task_id)'
"TypeError: unsupported operand type(s) for %: 'NoneType' and 'int'"
```

Signed-off-by: Martin Bašti <388356a3851efe39ff0604de6d8fb2e16e10a0b7@redhat.com>

fr34k8/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,DBuildService/atomic-reactor,projectatomic/atomic-reactor