mypy: Correct get*notifications_stream to return Optional[Stream].
mypy: Correct get*notifications_stream to return Optional[Stream].

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