feat(AIP-134): Make update_mask field must be optional (#1327)

* update aip-134 guidance to state update_mask field must be optional

* add changelog

Author: itsStrobe

aip/general/0134.md

@@ -100,10 +100,9 @@ message UpdateBookRequest {
   called `update_mask`.
   - The fields used in the field mask correspond to the resource being updated
     (not the request message).
-  - The field **may** be required or optional. If it is required, it **must**
-    include the corresponding annotation. If optional, the service **must**
-    treat an omitted field mask as an implied field mask equivalent to all
-    fields that are populated (have a non-empty value).
+  - The field **must** be optional, and the service **must** treat an omitted
+    field mask as an implied field mask equivalent to all fields that are
+    populated (have a non-empty value).
   - Update masks **must** support a special value `*`, meaning full replacement
     (the equivalent of `PUT`).
 - The request message **must not** contain any other required fields, and
@@ -298,6 +297,7 @@ does not exist, the service **must** error with `NOT_FOUND` (HTTP 404) unless
 
 ## Changelog
 
+- **2024-03-14**: Make `update_mask` optional field_behaviour guidance a **must**.
 - **2023-08-26**: Adding consistency requirement.
 - **2023-07-17**: Make `update_mask` name guidance a **must**.
 - **2022-11-04**: Aggregated error guidance to AIP-193.