notification bot: Refactor arguments to send_pm_if_empty_stream.
notification bot: Refactor arguments to send_pm_if_empty_stream.

Previously we sent "" for stream_name where we should have sent None, which
made this function harder to understand. The "" value was never used.

This also reorders the arguments to be match the order of the arguments in
the two callers.

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