make PaginatedView fetch next on empty collection
make PaginatedView fetch next on empty collection

Announcements index page was not fetching if the first page was empty
and the window was too short to trigger the scroll fetch.

fixes CNVS-26755

test plan:
* create 11 announcements with delayed posts set in the future
* as a student, view the announcements index page with a very short
  window (say 200px)
* the page should not get stuck on "Loading..." and should instead move
  on to say there are no announcements to show.

Change-Id: Ibe1ceca85668a0fbf1660d86ad8eb7a070a76750
Reviewed-on: https://gerrit.instructure.com/93765
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
Tested-by: Jenkins
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>
Product-Review: Jon Willesen <0321efbc0acf6204eb6035acb1d418838b450d08@instructure.com>

matematikk-mooc/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms