fix(widget-builder): Prevent TypeError when sorting with equation-only yAxis

[seer-by-sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'kind') that occurred in the widget builder when attempting to update the sort field.

Root Cause:
The error happened in useWidgetBuilderState.tsx within the SET_FIELDS action handler. Specifically, when a user removed a group-by field from a chart widget, the logic to determine the new sort field could pass undefined to generateFieldAsString().

This occurred because:

1. firstYAxisNotEquation would be undefined if the yAxis only contained fields of FieldValueKind.EQUATION.
2. firstActionPayloadNotEquation would also be undefined if the remaining action.payload (fields) were empty or only contained equations.
3. The code then attempted to call generateFieldAsString(undefined as QueryFieldValue), where the as QueryFieldValue cast silenced TypeScript but allowed undefined to be passed at runtime, leading to the TypeError when undefined.kind was accessed.

Solution:
The fix introduces explicit null checks before calling generateFieldAsString() or generateMetricAggregate().

• The sortField variable is now initialized as string | undefined.
• Conditional logic ensures that generateMetricAggregate() is called only if firstYAxisNotEquation is defined for TRACEMETRICS.
• Similarly, generateFieldAsString() is called only if firstYAxisNotEquation or firstActionPayloadNotEquation are defined.
• The setSort action is now guarded by an if (sortField) check, preventing a sort update (and thus the crash) if no valid field can be determined for sorting.

Closes JAVASCRIPT-35PG

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