title: Appsync Resolver UpdateItem ignore null args?
body: <p>I am doing this in my Appsync Resolver:</p>
  
  <pre><code>{
      "version" : "2017-02-28",
      "operation" : "UpdateItem",
      "key" : {
          "pk" : { "S" : "Container" },
          "id" : { "S" : "${ctx.args.id}" }
      },
      "update" : {
          "expression" : "SET #name = :name, description = :description",
          "expressionNames": {
              "#name" : "name"
          },
          "expressionValues": {
              ":name" : { "S": "${context.arguments.name}" },
              ":description" : { "S": "${context.arguments.description}" },
          }
      }
  }
  </code></pre>
  
  <p>But sometimes I may not pass in both name and description. How would I make it not SET those columns when those args are null?</p>