[FIX] ir.qweb: consider empty cached bundle as a cache miss

Sometimes a cached bundled could be missing in the
ir.attachment filestore (for example after copying
a database for test purposes without duplicating
the filestore as well).
When this happens ir.attachment will return an empty
file contents ; treat this as a cache miss.

This means empty bundles would not be cached, which
is not a big issue - there is little benefit in
caching them, and they should not be common nor
useful.