Incorrect type conversion incase of empty map

[SuperMohit]

Issue #, if available:

When the Response is an empty map, an incorrect type conversion is seen. As the caller is checking on nil, type returning the nil fixes the error.

Description of changes: Stack Trace:

{
  "errorMessage": "reflect.Set: value of type map[string]interface {} is not assignable to type string",
  "errorType": "string",
  "stackTrace": [
    {
      "path": "github.com/aws/aws-lambda-go@v1.13.3/lambda/function.go",
      "line": 35,
      "label": "(*Function).Invoke.func1"
    },
    {
      "path": "runtime/panic.go",
      "line": 884,
      "label": "gopanic"
    },
    {
      "path": "reflect/value.go",
      "line": 3145,
      "label": "Value.assignTo"
    },
    {
      "path": "reflect/value.go",
      "line": 2160,
      "label": "Value.Set"
    },
    {
      "path": "Users/mohittalniya/cloudformation-cli-go-plugin/cfn/encoding/stringify.go",
      "line": 103,
      "label": "Stringify"
    },

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.