message_list: Fix handling of unknown email in mentions.
message_list: Fix handling of unknown email in mentions.

If a user has an old mention and has since been renamed, there's
really nothing for us to do to render it; we should just return as
though we have no data.

eeshangarg/zulip,timabbott/zulip,rishig/zulip,timabbott/zulip,synicalsyntax/zulip,punchagan/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,dhcrzf/zulip,rishig/zulip,zulip/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,showell/zulip,brainwane/zulip,rishig/zulip,dhcrzf/zulip,rishig/zulip,hackerkid/zulip,eeshangarg/zulip,rishig/zulip,dhcrzf/zulip,kou/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,synicalsyntax/zulip,andersk/zulip,punchagan/zulip,dhcrzf/zulip,rht/zulip,rht/zulip,shubhamdhama/zulip,rht/zulip,tommyip/zulip,brainwane/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,eeshangarg/zulip,tommyip/zulip,hackerkid/zulip,brainwane/zulip,dhcrzf/zulip,brainwane/zulip,showell/zulip,zulip/zulip,showell/zulip,rishig/zulip,kou/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,dhcrzf/zulip,hackerkid/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,rht/zulip,timabbott/zulip,hackerkid/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,shubhamdhama/zulip,zulip/zulip,rishig/zulip,brainwane/zulip,showell/zulip,kou/zulip,punchagan/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,dhcrzf/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,tommyip/zulip,tommyip/zulip,timabbott/zulip