In certain cases, like when `SiteSetting.slug_generation_method`
is set to `none` with certain locales, the autogenerated chat
channel slugs will end up blank. This was causing errors in
unrelated jobs calling `update!` on the channel. Instead, we
should just copy Category behaviour, which does not error
if the autogenerated slug ends up blank. We already allow
for this with chat channel URLs, using `-` in place of the
missing slug.
07cc8c8634