Tracepoints monitor_qmp_cmd_in_band and
monitor_qmp_cmd_out_of_band (commit cf869d53 "qmp: support
out-of-band (oob) execution") treat non-string "id" like absent "id".
Fix that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-10-armbru@redhat.com>