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

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.