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

* Revert "revert 484e023c0"

This reverts commit c03b70c949.

* also allow pure renote

* fix checks for pure renote

packages/backend/src/models/schema/emoji.ts

@@ -27,6 +27,7 @@ export const packedEmojiSchema = {
 		host: {
 			type: 'string',
 			optional: false, nullable: true,
+			description: 'The local host is represented with `null`.',
 		},
 		url: {
 			type: 'string',

packages/backend/src/models/schema/user.ts

@@ -21,6 +21,7 @@ export const packedUserLiteSchema = {
 			type: 'string',
 			nullable: true, optional: false,
 			example: 'misskey.example.com',
+			description: 'The local host is represented with `null`.',
 		},
 		avatarUrl: {
 			type: 'string',