stream settings: Fix exceptions when rerendering subscribers.
stream settings: Fix exceptions when rerendering subscribers.

If another user subscribed to or unsubscribed from a stream while the
current user was not subscribed, we previously would attempt to
rerender the subscriber counts for that stream, even though they
weren't rendered at all in the first place; this would trigger
blueslip errors from the expectOne() check in this function.

Fixes #8720.

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