prevent sending to an empty course section

test plan:
 -  in a course with an empty section

 1) compose a new conversation
 2) click the address book
 3) select the course > sections > the empty section
 4) there should not be an option to select 'All in ...'
 5) repeat steps 2-4 (without refreshing the page)

 6) choose a valid recipient (not the empty section)
 7) complete and send the message
 8) the 'sending...' indicator should show for a moment, then disappear

fixes CNVS-10851

Change-Id: I991d6547a94ea7e69c368dce909f29e080e2b14b
Reviewed-on: https://gerrit.instructure.com/39685
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
Reviewed-by: Alex Boyd <aboyd@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>