Avoid blueslip error for empty streams.
Avoid blueslip error for empty streams.

We simplify the code for deciding whether
we show a subscribe button or not, and in
doing so avoid a blueslip error where we
were passing `undefined` into `get_sub()`.

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