[dashboard v2] add <MissingChart /> component

[williaster]

This PR adds a <MissingChart /> component that is shown in the case that a dashboard v2 <Chart /> component has no slice definition.

Testing

• verified that this works in the case that a layout references a chart that was deleted (and previously part of the dashboard)
• Added frontend tests for <MissingChart />
• Tested undo/redo of removing the empty chart, adds toast when we add a slice that has no definition to a dashboard

@graceguo-supercat

[codecov-io]

Codecov Report

Merging #5296 into master will increase coverage by 0.7%.
The diff coverage is 86.66%.

@@           Coverage Diff            @@
##           master   #5296     +/-   ##
========================================
+ Coverage   61.29%     62%   +0.7%     
========================================
  Files         368     369      +1     
  Lines       23457   23981    +524     
  Branches     2716    2717      +1     
========================================
+ Hits        14379   14870    +491     
- Misses       9066    9099     +33     
  Partials       12      12

Impacted Files	Coverage Δ
...set/assets/src/dashboard/actions/dashboardState.js	41.98% <0%> (-0.66%)	⬇️
...erset/assets/src/SqlLab/components/QuerySearch.jsx	68.18% <100%> (ø)	⬆️
.../src/dashboard/components/gridComponents/Chart.jsx	83.13% <100%> (+0.41%)	⬆️
...t/assets/src/dashboard/components/MissingChart.jsx	100% <100%> (ø)
superset/viz.py	84.55% <0%> (+3.16%)	⬆️

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[graceguo-supercat]

LGTM