mod_rest: Allow empty @to to mean to=account is in normal XMPP Noticed an XXX in snikket-web-portal wrt needing to explicitly set @to in order to talk to 'self' services in Prosody, where to=nil is the same as to=account

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.