fix(widget-builder): Prevent crash when threshold max_values is undefined

[sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'max1') occurring in the widget builder's thresholds step.

The root cause was identified in static/app/views/dashboards/widgetBuilder/buildSteps/thresholdsStep/thresholds.tsx. Lines 105-106 attempted to access thresholdsConfig?.max_values[ThresholdMaxKeys.MAX_1] (and MAX_2). While thresholdsConfig was optionally chained, max_values itself was not. If thresholdsConfig existed but max_values was undefined (which can happen despite the TypeScript type declaring it as required), the bracket access undefined['max1'] would throw a TypeError.

The fix involves adding optional chaining (?.) to the max_values property access, changing thresholdsConfig?.max_values[key] to thresholdsConfig?.max_values?.[key]. This ensures that if max_values is undefined, the expression short-circuits, and the ?? '' fallback correctly provides an empty string, preventing the crash.

A regression test has been added to thresholds.spec.tsx to cover the scenario where max_values is undefined, ensuring the component renders correctly without errors.

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Fixes JAVASCRIPT-3BYC

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Story previews

Preview the stories changed in this PR on the Vercel deployment:

• app/views/dashboards/widgets/bigNumberWidget/bigNumberWidgetVisualization.stories.tsx

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