title: GraphQL-js mutation optional arguments
body: <p>how would I achieve a GraphQL Mutation in nodeJS that has arguments which are optional?</p>
  
  <p>My current mutation has an args field, but all the arguments are mandatory. Since I couldn't find anything in the documentation, I don't know how or if this is possible.</p>
  
  <p>This is my current code:</p>
  
  <pre><code>const fakeDB = {
      count: 0
  };
  
  const schema = new GraphQLSchema({
      query: //...
      mutation: new GraphQLObjectType({
          name: 'adsF',
          fields: {
              updateCount: {
                  type: GraphQLInt,
                  args: {
                      count: { type: GraphQLInt } // I want to make this argument optional
                  },
                  resolve: (value, { count }) =&gt; {
                      // Catch if count is null or undefined
                      if (count == null) {
                          // If so just update with default
                          fakeDB.count = 5;
                      } else {
                          fakeDB.count = count
                      }
                      return fakeDB.count;
                  })
              }
          }
      })
  });
  </code></pre>
  
  <p>Thanks for Your help!</p>