fix: fix assignment in FilterBoxViz

[tianhe1986]

SUMMARY

Just a small fix, assigning to the right object.
Or, it will throw TypeError: 'NoneType' object does not support item assignment when df is None.

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

[villebro]

LGTM; thanks for the fix!

[codecov]

Codecov Report

Merging #16662 (b2ad3f0) into master (6465ee7) will increase coverage by 0.07%.
The diff coverage is 83.82%.

❗ Current head b2ad3f0 differs from pull request most recent head c772fd5. Consider uploading reports for the commit c772fd5 to get more accurate results

@@            Coverage Diff             @@
##           master   #16662      +/-   ##
==========================================
+ Coverage   76.70%   76.78%   +0.07%     
==========================================
  Files        1004     1005       +1     
  Lines       53983    54005      +22     
  Branches     7335     7337       +2     
==========================================
+ Hits        41408    41467      +59     
+ Misses      12336    12298      -38     
- Partials      239      240       +1     

Flag	Coverage Δ
hive	?
mysql	81.65% <81.81%> (+0.06%)	⬆️
postgres	81.68% <81.81%> (+0.06%)	⬆️
presto	81.52% <81.81%> (?)
python	81.94% <81.81%> (-0.04%)	⬇️
sqlite	81.33% <81.81%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
...mponents/nativeFilters/FiltersConfigModal/utils.ts	72.22% <ø> (-1.39%)	⬇️
...nd/src/dashboard/components/nativeFilters/utils.ts	56.25% <ø> (ø)
superset/sql_lab.py	80.70% <ø> (ø)
superset/views/core.py	76.00% <81.48%> (+0.37%)	⬆️
superset-frontend/src/components/Select/Select.tsx	89.95% <88.88%> (+17.30%)	⬆️
...onfigModal/FiltersConfigForm/FiltersConfigForm.tsx	73.20% <100.00%> (+0.07%)	⬆️
...nd/src/dashboard/components/nativeFilters/types.ts	100.00% <100.00%> (ø)
superset/viz.py	57.42% <100.00%> (+0.79%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.80% <0.00%> (-16.87%)	⬇️
... and 10 more

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