private/model: Handles empty map vs unset map behavior in send request (#337)

Updated shape marshal model to handle the empty map vs nil map behavior.
Adding a test case to assert behavior when a user sends an empty map vs nil map.

Fix #332

Author: skotambkar

CHANGELOG_PENDING.md

@@ -3,3 +3,6 @@
 ### SDK Enhancements
 
 ### SDK Bugs
+* `private/model`: Handles empty map vs unset map behavior in send request ([#337](https://github.com/aws/aws-sdk-go-v2/pull/337))
+  * Updates shape marshal model to handle the empty map vs nil map behavior. Also adds a test case to assert behavior when a user sends an empty map vs unset map.
+  * Fixes [#332](https://github.com/aws/aws-sdk-go-v2/issues/332)

models/protocol_tests/input/rest-json.json

@@ -1657,5 +1657,65 @@
         }
       }
     ]
+  },
+  {
+    "description": "Empty or unset maps in payload",
+    "metadata": {
+      "protocol": "rest-json",
+      "apiVersion": "2014-01-01"
+    },
+    "shapes": {
+      "InputShape": {
+        "type": "structure",
+        "members": {
+          "MapEmpty": {
+            "shape": "MapStringStringType"
+          },
+          "MapDefaults": {
+            "shape": "MapStringStringType"
+          },
+          "MapNotSet": {
+            "shape": "MapStringStringType"
+          }
+        }
+      },
+      "MapStringStringType": {
+        "type": "map",
+        "key": {
+          "shape": "StringType"
+        },
+        "value": {
+          "shape": "StringType"
+        }
+      },
+      "StringType": {
+        "type": "string"
+      }
+    },
+    "cases": [
+      {
+        "given": {
+          "http": {
+            "method": "POST",
+            "requestUri": "/"
+          },
+          "input": {
+            "shape": "InputShape"
+          },
+          "name": "OperationName"
+        },
+        "params": {
+          "MapEmpty": {},
+          "MapDefaults": {"test_key_1": "test_value_1", "test_key_2": "test_value_2" ,"test_key_3": "test_value_3" }
+        },
+        "serialized": {
+          "body": "{\"MapDefaults\": {\"test_key_1\":\"test_value_1\", \"test_key_2\":\"test_value_2\", \"test_key_3\":\"test_value_3\"}, \"MapEmpty\":{}}",
+          "uri": "/",
+          "headers": {}
+        }
+      }
+    ]
   }
 ]
+
+

private/model/api/shape_marshal.go

@@ -292,7 +292,7 @@ var marshalShapeRefTmpl = template.Must(template.New("marshalShapeRefTmpl").Func
 	{{ $isList := $.IsShapeType "list" -}}
 	{{ $isMap := $.IsShapeType "map" -}}
 	{{- if or $isList $isMap -}}
-		len(s.{{ $.Name }}) > 0
+		s.{{ $.Name }} != nil
 	{{- else if $.Ref.Shape.IsEnum -}}
 		len(s.{{ $.Name }}) > 0
 	{{- else -}}