Unset background when switching to a non-background story

[Hypnosphi]

Issue: After #2807, backgrounds keep being present when switching to a non-background story

How to test

Open a story with backgrounds, then navigate to a story without them. Backgrounds should disappear

[codecov]

Codecov Report

Merging #3142 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3142      +/-   ##
==========================================
+ Coverage   36.15%   36.16%   +<.01%     
==========================================
  Files         437      437              
  Lines        9429     9430       +1     
  Branches      883      899      +16     
==========================================
+ Hits         3409     3410       +1     
+ Misses       5470     5453      -17     
- Partials      550      567      +17

Impacted Files	Coverage Δ
addons/background/src/BackgroundPanel.js	92.5% <100%> (+0.19%)	⬆️
app/vue/src/server/utils.js	0% <0%> (-100%)	⬇️
app/react/src/client/preview/element_check.js	67.64% <0%> (ø)	⬆️
...i/src/modules/ui/components/stories_panel/index.js	21.66% <0%> (ø)	⬆️
lib/ui/src/modules/ui/libs/filters.js	48.64% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
lib/codemod/src/transforms/update-addon-info.js	50% <0%> (ø)	⬆️
addons/knobs/src/components/types/Button.js	11.9% <0%> (ø)	⬆️
...tions/src/lib/types/function/createFunctionEval.js	77.77% <0%> (ø)	⬆️
...tive/src/preview/components/StoryListView/index.js	0% <0%> (ø)	⬆️
... and 60 more

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