chore(dashboard): Ignore empty json value for overwrite confirm

[justinpark]

SUMMARY

Minor regression from #21819
When user change a single layout but no filter_scope updates, there's unrelated overwrite confirmation kept popping up because the default json value is not empty string but an empty object {}.

This commit updates the default json value to skip the unnecessary overwrite warning.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:

ignore-empty-json-overwrite.mov

After:

No overwrite confirm

TESTING INSTRUCTIONS

• Enable CONFIRM_DASHBOARD_DIFF
• Edit dashboard and change one chart position
• Save and check the confirmation

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

cc: @ktmud @john-bodley

[codecov]

Codecov Report

Merging #22214 (265564c) into master (888f43c) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #22214      +/-   ##
==========================================
- Coverage   66.96%   66.92%   -0.04%     
==========================================
  Files        1835     1835              
  Lines       69927    69988      +61     
  Branches     7590     7612      +22     
==========================================
+ Hits        46826    46839      +13     
- Misses      21135    21183      +48     
  Partials     1966     1966              

Flag	Coverage Δ
javascript	53.75% <50.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
...t-frontend/src/dashboard/util/getOverwriteItems.ts	90.90% <50.00%> (ø)
...ntend/plugins/plugin-chart-echarts/src/defaults.ts	20.00% <0.00%> (-80.00%)	⬇️
.../src/explore/components/ControlPanelsContainer.tsx	75.35% <0.00%> (-2.03%)	⬇️
...tend/plugins/plugin-chart-echarts/src/Pie/types.ts	100.00% <0.00%> (ø)
...tend/plugins/plugin-chart-echarts/src/constants.ts	100.00% <0.00%> (ø)
...nd/plugins/plugin-chart-echarts/src/Graph/types.ts	100.00% <0.00%> (ø)
...nd/plugins/plugin-chart-echarts/src/Radar/types.ts	100.00% <0.00%> (ø)
...d/plugins/plugin-chart-echarts/src/Funnel/types.ts	100.00% <0.00%> (ø)
.../plugins/plugin-chart-echarts/src/BoxPlot/types.ts	0.00% <0.00%> (ø)
.../plugins/plugin-chart-echarts/src/Treemap/types.ts	100.00% <0.00%> (ø)
... and 29 more

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[villebro]

LGTM