mypy: Fix mypy error with optional.
mypy: Fix mypy error with optional.

This was missed in a57d3371135ae13b531525a79a6c6e55905d95e2.

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