fix: define Mutation type in GraphQL schema to ensure proper structure

Author: productdevbook

playground-nuxt/server/graphql/schema.graphql

@@ -6,4 +6,6 @@ type Query {
   greeting(name: String!): String!
 }
 
-type Mutation
+type Mutation {
+  _empty: String
+}

playground/server/graphql/schema.graphql

@@ -6,4 +6,6 @@ type Query {
   greeting(name: String!): String!
 }
 
-type Mutation
+type Mutation {
+  _empty: String
+}