feat: add onEmpty callback to Command

[stevysmith]

Summary

• Adds onEmpty prop to Command — an event handler called when the search query produces no results
• Fires with the current search string, only when there's a non-empty query and zero matching items
• Follows the existing callback pattern (onValueChange, onSelect) using optional chaining via propsRef

Motivation

There's no way to react programmatically when filtering produces zero results. Command.Empty handles the visual side, but there's no behavioral hook for the event itself. This is useful for:

• Analytics — capture unmatched queries to understand what users are looking for
• AI fallback — trigger an alternative search or AI-powered response when results are empty
• Feedback collection — prompt users to submit a request when their search yields nothing

Usage

<Command
  onEmpty={(search) => {
    console.log('No results for', search)
  }}
/>