fix: persist a valid JSON fallback when removing bindings (#6492)

## What changed

Resetting a JSON binding stores the expression's evaluated value. If it
evaluates to `undefined`, the serialized prop loses its `value` field
and fails the build schema. This change saves `null` instead for a
nullish fallback.

## Reproduction

Using the SDK `prop` schema locally, a JSON prop with the `value` field
omitted fails validation; adding `value: null` passes.

The full test suite was not run.

apps/builder/app/builder/features/settings-panel/controls/json.tsx

@@ -55,7 +55,7 @@ export const JsonControl = ({
         value={computedValue}
         onChangeValue={(value) => onChange({ type: "json", value })}
         onChangeExpression={(value) => onChange({ type: "expression", value })}
-        onRemove={(value) => onChange({ type: "json", value })}
+        onRemove={(value) => onChange({ type: "json", value: value ?? null })}
         renderControl={({ readOnly }) => (
           <ExpressionEditor
             color={error ? "error" : undefined}