stream settings: Fix brand new streams showing 0 weekly traffic.
stream settings: Fix brand new streams showing 0 weekly traffic.

A stream created in the last few hours likely won't be in StreamCount
(since that gets updated once a day), and hence won't be in the
recent_traffic dict.

However, get_average_weekly_stream_traffic should be None in this case,
not 0.

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