fix: don't anticipate i18n a11y object will be present

I forgot users can provide their own i18n, in which case it can come to
TypeErrors. So I shouldn't count on the a11y always being there.

Relates to PS-21279.

packages/emoji-mart/src/components/Picker/Picker.tsx

@@ -1180,7 +1180,7 @@ export default class Picker extends Component {
               width: this.props.dynamicWidth ? '100%' : lineWidth,
               height: '100%',
             }}
-            aria-label={I18n.a11y.available_emojis ?? 'Available emojis'}
+            aria-label={I18n.a11y?.available_emojis ?? 'Available emojis'}
           >
             {this.props.searchPosition == 'static' && this.renderSearch()}
             {this.renderSearchResults()}