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.
syntax: glob
.hgignore
.luacheckcache
data
local
www_files
html/*
prosody.lua
prosody.cfg.lua
prosody.version
config.unix
*.patch
*.diff
*.orig
*.rej
*.save
*~
*.o
*.so
*.install
*.pid
*.log
*.err
*.debug
*.dll
*.exp
*.lib
*.obj
luacov.report.out
luacov.report.out.index
luacov.stats.out