fix caching of nil PluginSettings
fix caching of nil PluginSettings

test plan:
 * enable caching
 * load a page a few times
 * verify that PluginSetting isn't queried each time (in the logs)

Change-Id: Id49f2001c4077f3351d96957c8681c7e91aec30c
Reviewed-on: https://gerrit.instructure.com/13595
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
Tested-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

skmezanul/canvas-lms,juoni/canvas-lms,venturehive/canvas-lms,usmschuck/canvas,jay3126/canvas-lms,landonwilkins/canvas-lms,arrivu/hoodemo,usmschuck/canvas,sfu/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,antoniuslin/canvas-lms,arrivu/jigsaw-lms,wye0220/shindig-canvas,venturehive/canvas-lms,rup/Tarrax,wimemx/Canvas,Rvor/canvas-lms,Unow/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,arrivu/arrivu-lms,samuels410/greatlakes-lms,sdeleon28/canvas-lms,wye0220/shindig-canvas,skmezanul/canvas-lms,skmezanul/canvas-lms,ShreniiNepal/shrenii_lmp,juoni/canvas-lms,loganrun/pincan,sfu/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,antoniuslin/canvas-lms,usmschuck/canvas,jay3126/canvas-lms,arrivu/arrivu-lms,arrivu/hoodemo,Rvor/canvas-lms,fronteerio/canvas-lms,wrdsb/canvas-lms,greyhwndz/canvas-lms,nditech/red-innovadores-lms,samuels410/greatlakes-lms,shidao-fm/canvas-lms,parbros/red-innovadores-lms,arrivu/hoodemo,loganrun/pincan,arrivu/qibla-ecampus,va7map/canvas-lms,sfu/canvas-lms,sdeleon28/canvas-lms,landonwilkins/canvas-lms,efrj/canvas-lms,whalejasmine/canvas-lms,usmschuck/canvas,arrivu/jigsaw-lms,leftplusrightllc/canvas-lms,rup/Tarrax,tgroshon/canvas-lms,sli7246/canvas_branch,Rvor/canvas-lms,wimemx/Canvas,whalejasmine/canvas-lms,kyroskoh/canvas-lms,shidao-fm/canvas-lms,landonwilkins/canvas-lms,Unow/canvas-lms,nditech/red-innovadores-lms,grahamb/canvas-lms,sfu/canvas-lms,arrivu/jigsaw-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,tgroshon/canvas-lms,antoniuslin/canvas-lms,efrj/canvas-lms,samuels410/greatlakes-lms,HotChalk/canvas-lms,va7map/canvas-lms,sfu/canvas-lms,arrivu/qibla-ecampus,parbros/red-innovadores-lms,efrj/canvas-lms,antoniuslin/canvas-lms,Unow/canvas-lms,sdeleon28/canvas-lms,greyhwndz/canvas-lms,arrivu/qibla-ecampus,wimemx/Canvas,grahamb/canvas-lms,djbender/canvas-lms,va7map/canvas-lms,Jyaasa/canvas-lms,ankithbti/canvas_lms,tgroshon/canvas-lms,wrdsb/canvas-lms,nditech/red-innovadores-lms,ShreniiNepal/shrenii_lmp,parbros/red-innovadores-lms,ShreniiNepal/shrenii_lmp,fronteerio/canvas-lms,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,fronteerio/canvas-lms,arrivu/arrivu-lms,Jyaasa/canvas-lms,leftplusrightllc/canvas-lms,merichar/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,rup/Tarrax,va7map/canvas-lms,djbender/canvas-lms,Unow/ins,whalejasmine/canvas-lms,utkarshx/canvas-lms,samuels410/greatlakes-lms,kyroskoh/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,juoni/canvas-lms,kyroskoh/canvas-lms,landonwilkins/canvas-lms,Jyaasa/canvas-lms,wye0220/shindig-canvas,utkarshx/canvas-lms,redconfetti/canvas-lms,kyroskoh/canvas-lms,juoni/canvas-lms,ankithbti/canvas_lms,shidao-fm/canvas-lms,leftplusrightllc/canvas-lms,sli7246/canvas_branch,utkarshx/canvas-lms,greyhwndz/canvas-lms,efrj/canvas-lms,shidao-fm/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,Jyaasa/canvas-lms,roxolan/canvas-lms,sdeleon28/canvas-lms,venturehive/canvas-lms,loganrun/pincan,nditech/red-innovadores-lms,merichar/canvas-lms,redconfetti/canvas-lms,ankithbti/canvas_lms,sli7246/canvas_branch,Rvor/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,merichar/canvas-lms,utkarshx/canvas-lms,sfu/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,Unow/ins,roxolan/canvas-lms,HotChalk/canvas-lms,whalejasmine/canvas-lms,wrdsb/canvas-lms,ShreniiNepal/shrenii_lmp,skmezanul/canvas-lms,Unow/ins,jay3126/canvas-lms