Make request bodies optional (Azure#2674)

Still needs:

- [x] Sample
- [x] Tests
- [x] azure-rest-api-specs-prs

---------

Co-authored-by: Timothee Guerin <[email protected]>

Author: markcowl

.chronus/changes/opt-body-2025-4-12-19-18-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-azure-resource-manager"
+---
+
+Allow optional request bodies in Resource actions templates

packages/azure-http-specs/spec-summary.md

@@ -333,7 +333,7 @@ Expected response body:
 
 ### Azure_Core_Basic_createOrUpdate
 
-- Endpoint: `get /azure/core/basic`
+- Endpoint: `patch /azure/core/basic`
 
 Should only generate models named User and UserOrder.
 

packages/samples/specs/resource-manager/legacy/optional-body/main.tsp

@@ -0,0 +1,49 @@
+import "@typespec/rest";
+import "@typespec/http";
+import "@azure-tools/typespec-azure-resource-manager";
+import "@typespec/versioning";
+
+using Azure.ResourceManager;
+using Versioning;
+using Http;
+
+@armProviderNamespace
+@versioned(Versions)
+namespace Microsoft.OptionalBody;
+interface Operations extends Azure.ResourceManager.Operations {}
+
+enum Versions {
+  @useDependency(Azure.ResourceManager.Versions.v1_0_Preview_1)
+  @armCommonTypesVersion(Azure.ResourceManager.CommonTypes.Versions.v5)
+  `2025-01-01-preview`,
+}
+/** The resource-specific properties of a Widget */
+model WidgetProperties {
+  /** Required property */
+  property1: string;
+
+  /** optional Property */
+  property2: int32;
+
+  ...DefaultProvisioningStateProperty;
+}
+
+model Widget is TrackedResource<WidgetProperties> {
+  ...ResourceNameParameter<Widget>;
+}
+
+@armResourceOperations
+interface Widgets {
+  get is ArmResourceRead<Widget>;
+  createOrReplace is ArmResourceCreateOrReplaceAsync<Widget>;
+  update is Azure.ResourceManager.Legacy.CustomPatchAsync<
+    Widget,
+    Foundations.ResourceUpdateModel<Widget, WidgetProperties>,
+    RequestBody = {
+      /** The PATCH request body */
+      @body body?: Foundations.ResourceUpdateModel<Widget, WidgetProperties>;
+    }
+  >;
+  delete is ArmResourceDeleteWithoutOkAsync<Widget>;
+  list is ArmResourceListByParent<Widget>;
+}