title: Make mongoose string schema type default value as blank and make the field optional
body: <p>I have a test schema with mongoose in nodejs like</p>
  
  <pre><code>testschema = mongoose.Schema({
           name:{
      type:String,
      required:true,
      unique:true
    },
    image:{
      type:String,
      required:true
    },
    category:{
      type:String
    },
  });
  </code></pre>
  
  <p>How can i make the category field as optional and make it default to blank if not given by user? </p>
  
  <p>I tried </p>
  
  <pre><code> category:{
          type:String,
          optional: ''
        },
  </code></pre>
  
  <p>but when printing out the documents saved with the scheme it doesnt even shows the field category.</p>