fix: Properly encode empty bodies for json and xml (#480)

* Removes logic for case where the payload member is nil

* Fixes tests after adjusting HttpBodyMiddleware

* Correctly renders empty bodies

Author: Ed Paulosky

smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/handlers/HttpBodyMiddleware.kt

@@ -111,10 +111,19 @@ class HttpBodyMiddleware(
                         renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
                     }
                     writer.indent()
-                    writer.write("let $dataDeclaration = try encoder.encode(input.operationInput)")
+                    writer.openBlock("if encoder is JSONEncoder {", "} else if encoder is XMLEncoder {") {
+                        writer.write("// Encode an empty body as an empty structure in JSON")
+                        writer.write("let \$L = \"{}\".data(using: .utf8)!", dataDeclaration)
+                        renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
+                    }
+                    writer.indent()
+                    writer.write("// Encode an empty body as an empty string in XML")
+                    writer.write("let \$L = \"\".data(using: .utf8)!", dataDeclaration)
                     renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
                     writer.dedent()
                     writer.write("}")
+                    writer.dedent()
+                    writer.write("}")
                 }
                 renderErrorCase()
             }

smithy-swift-codegen/src/test/kotlin/HttpBodyMiddlewareTests.kt

@@ -155,9 +155,17 @@ class HttpBodyMiddlewareTests {
                             let payload1body = ClientRuntime.HttpBody.data(payload1data)
                             input.builder.withBody(payload1body)
                         } else {
-                            let payload1data = try encoder.encode(input.operationInput)
-                            let payload1body = ClientRuntime.HttpBody.data(payload1data)
-                            input.builder.withBody(payload1body)
+                            if encoder is JSONEncoder {
+                                // Encode an empty body as an empty structure in JSON
+                                let payload1data = "{}".data(using: .utf8)!
+                                let payload1body = ClientRuntime.HttpBody.data(payload1data)
+                                input.builder.withBody(payload1body)
+                            } else if encoder is XMLEncoder {
+                                // Encode an empty body as an empty string in XML
+                                let payload1data = "".data(using: .utf8)!
+                                let payload1body = ClientRuntime.HttpBody.data(payload1data)
+                                input.builder.withBody(payload1body)
+                            }
                         }
                     } catch let err {
                         throw SdkError<ExplicitStructOutputError>.client(ClientRuntime.ClientError.serializationFailed(err.localizedDescription))