fix: show 'no data' in KPI component when measure value is null

[ericokuma]

When the KPI query succeeds but returns null data (e.g. no matching rows for the selected filters/time range), the measureValueFormatter is called with null. Since the formatter is typed as createMeasureValueFormatter<null>, it passes null through unchanged, and Svelte renders null as empty text — resulting in a blank measure value.

Changes

• Primary value: Added a null check before formatting. When the value is null, shows italic "no data" text in muted styling instead of a blank space.
• Comparison "previous" value: Added inline null check to show "no data" text when the comparison value is null.

This matches the existing pattern used in the explore dashboard's MeasureBigNumber.svelte component, which already displays "no data" for null values (line 327-328).

Closes APP-858

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

Linear Issue: APP-858