mod_storage_xep0227: be defensive against empty vCard
An empty vCard store may look like the empty table, which does not have
the `attr` key, which would then blow up in util.stanza.deserialize.
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec