fix: safe check when computing chart thumbnail

[zephyring]

SUMMARY

1. safe check if chart is null before actually computing the thumbnail

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #23432 (3060955) into master (3277ea8) will increase coverage by 1.60%.
The diff coverage is 76.19%.

❗ Current head 3060955 differs from pull request most recent head 227f1c2. Consider uploading reports for the commit 227f1c2 to get more accurate results

@@            Coverage Diff             @@
##           master   #23432      +/-   ##
==========================================
+ Coverage   66.03%   67.63%   +1.60%     
==========================================
  Files        1908     1908              
  Lines       73722    73729       +7     
  Branches     7989     7989              
==========================================
+ Hits        48682    49867    +1185     
+ Misses      22992    21814    -1178     
  Partials     2048     2048              

Flag	Coverage Δ
hive	52.73% <33.33%> (-0.01%)	⬇️
mysql	78.44% <33.33%> (-0.01%)	⬇️
postgres	78.50% <33.33%> (-0.01%)	⬇️
presto	52.65% <33.33%> (-0.01%)	⬇️
python	82.36% <75.00%> (+3.36%)	⬆️
sqlite	77.01% <33.33%> (-0.01%)	⬇️
unit	52.58% <75.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/components/Chart/DrillDetail/DrillDetailPane.tsx	75.00% <ø> (ø)
...tersConfigModal/FiltersConfigForm/ColumnSelect.tsx	77.14% <ø> (ø)
...onfigModal/FiltersConfigForm/FiltersConfigForm.tsx	55.17% <ø> (-4.49%)	⬇️
.../dataset/AddDataset/EditDataset/UsageTab/index.tsx	90.47% <ø> (ø)
superset/config.py	91.78% <ø> (ø)
superset/tasks/thumbnails.py	36.36% <0.00%> (-2.67%)	⬇️
superset-frontend/src/components/Table/index.tsx	71.42% <60.00%> (ø)
...set-frontend/src/components/Table/VirtualTable.tsx	72.30% <100.00%> (ø)
...c/views/CRUD/data/database/DatabaseModal/index.tsx	45.47% <100.00%> (ø)
superset/common/query_object.py	95.97% <100.00%> (+1.62%)	⬆️
... and 1 more

... and 90 files with indirect coverage changes

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