react [nfc]: Remove a useless Fragment

I caught this while experimenting with a new(ish) eslint rule:
  react/jsx-no-useless-fragment
  https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-no-useless-fragment.md

src/streams/TopicItem.js

@@ -106,7 +106,7 @@ export default function TopicItem(props: Props): Node {
           numberOfLines={1}
           ellipsizeMode="tail"
         />
-        {isMentioned ? <IconMention size={14} style={componentStyles.mentionedLabel} /> : <></>}
+        {isMentioned && <IconMention size={14} style={componentStyles.mentionedLabel} />}
         <UnreadCount count={unreadCount} inverse={isSelected} />
       </View>
     </Touchable>