Fix empty layers spinning forever in Multi Map legend

[ethanbienstock]

Summary

• Differentiate between loading and empty layer states in the Multi Map legend so that layers with no data stop showing an infinite spinner
• Add a gray "Visible but Empty" label below the group title for empty layers, aligned with the group header
• Gray out the checkbox accent color for empty layer entries
• Add 6 tests covering the new empty layer behavior

Closes #350

Focus Score

9/10 — All changes are tightly scoped to one bug: empty layers causing an infinite spinner in the Multi Map legend. The only files touched are the legend component, its test file, the layer processing logic in Multi.tsx, and the shared type definition. The formatting commit is a minor tangent but affects the same file.

Detailed Summary of Each File Changed

superset-frontend/plugins/geoset-map-chart/src/GeoSetMultiMap/Multi.tsx

• Removed the early return null when getGeoSetMapLayer returns null (empty data)
• Changed layerStatesGenerator call to a ternary that returns [] when newLayer is falsy
• Changed the !newLayerStates.length branch from return null to setting legendEntry.empty = true, so the entry still reaches the legend

superset-frontend/plugins/geoset-map-chart/src/components/MultiLegend.tsx

• Added NoDataLabel styled component (gray, 11px, aligned with group title via margin-left: -20px)
• Renders "Visible but Empty" text when legendEntry.empty is true
• Added $empty transient prop to VisibilityCheckbox and IndeterminateCheckbox to apply accent-color: gray styling
• Computed allEmpty at the group level for the group header checkbox

superset-frontend/plugins/geoset-map-chart/src/types.ts

• Added empty?: boolean field to the LegendEntry type

superset-frontend/plugins/geoset-map-chart/test/components/MultiLegend.test.tsx

• Added 6 tests in a new empty layer state describe block:
  1. Empty entry shows checkbox + "Visible but Empty" label (not spinner)
  2. Non-empty entry does not show the label
  3. Group header shows checkbox when all entries are empty
  4. Group header checkbox present when mix of empty/non-empty
  5. Loading takes precedence over empty (spinners shown)
  6. Transition from loading to empty replaces spinner with label

Test Plan

• Automated: Run npx jest --config jest.config.js plugins/geoset-map-chart/test/components/MultiLegend.test.tsx — all 30 tests should pass
• Manual: Open the Hurricane dashboard with year filter set to 2026 (no data). Verify the legend shows "Visible but Empty" in gray text instead of an infinite spinner
• Manual: Switch the year filter to a year with data (e.g. 2024). Verify the spinner transitions to the normal legend content
• Manual: With multiple layer groups, verify the group-level checkbox turns gray when all entries in the group are empty