Current OfflineMessageStore logic discards valid MUC invites

OfflineMessageStore currently discards any messages with an empty body.

An exception is made for PubSub events (See ) as (I assume) these don't need a body to be useful to the recipient.

Another example of a valid message which doesn't have a body is a MUC invite.

e.g.

<message
from='crone1@shakespeare.lit/desktop'
to='hecate@shakespeare.lit'>
<x xmlns='jabber:x:conference'
jid='darkcave@macbeth.shakespeare.lit'
password='cauldronburn'
reason='Hey Hecate, this is the place for all good witches!'/>
</message>

These messages are currently discarded when the user is offline - instead, they should be stored according to the current offline message storage strategy