Fix emoji search not showing custom emoji when none are uncategorized (#11920)

Fix #11903

app/javascript/mastodon/features/emoji/emoji.js

@@ -99,4 +99,4 @@ export const buildCustomEmojis = (customEmojis) => {
   return emojis;
 };
 
-export const categoriesFromEmojis = customEmojis => customEmojis.reduce((set, emoji) => set.add(emoji.get('category') ? `custom-${emoji.get('category')}` : 'custom'), new Set());
+export const categoriesFromEmojis = customEmojis => customEmojis.reduce((set, emoji) => set.add(emoji.get('category') ? `custom-${emoji.get('category')}` : 'custom'), new Set(['custom']));