stream: Fix message on clicking add button with empty subscribers form.
stream: Fix message on clicking add button with empty subscribers form.

This commit corrects the message shown when we click the add button
for subscribing users to stream with empty input.
We show 'No user to subscribe.' as the message when trying to add
subscribers with empty input.

Fixes #15450.

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