fix: validation (better #8456) (#8461)

* Revert "revert 484e023c0"

This reverts commit c03b70c949.

* also allow pure renote

* fix checks for pure renote

packages/backend/src/server/api/endpoints/admin/emoji/update.ts

@@ -23,7 +23,11 @@ export const paramDef = {
 	properties: {
 		id: { type: 'string', format: 'misskey:id' },
 		name: { type: 'string' },
-		category: { type: 'string', nullable: true },
+		category: {
+			type: 'string',
+			nullable: true,
+			description: 'Use `null` to reset the category.',
+		},
 		aliases: { type: 'array', items: {
 			type: 'string',
 		} },