Don't slugify() None names

If image has no name, human_id can't be built.
Also slugify raises ValueError if None argument passed.
It affects Docker images.

Closes-Bug: #1267429
Closes-Bug: #1267130
Change-Id: Ib975775b441917eef2a650049cee9991d10c50d7
Merge "Don't slugify() None names"