fix(AIP-4235): update generator behavior for explicit field presence (#1325)

* fix: update 4235 to include explicit field presence

* update

* reword to positive

Author: alicejli

aip/client-libraries/4235.md

@@ -7,7 +7,7 @@ created: 2024-03-07
 # Automatically populate fields in the request message
 
 For APIs that leverage request idempotency as described via [AIP-155], APIs may
-choose to have the client libraries automatically populate fields such as 
+choose to have the client libraries automatically populate fields such as
 `request_id` if they are not already set by the customer.
 
 **Note:** This feature is primarily written for `request_id` fields within the
@@ -38,14 +38,21 @@ If the aforementioned requirements are met for a given field, client library
 generators **must** enable automatic population of said field in the
 generated client.
 
+The field **must** be automatically populated if and only if one of the
+following conditions holds:
+
+- The field supports explicit presence, and has **not** been set by the user
+- The field doesn't support explicit presence, and its value is the empty
+  string (i.e. the default value)
+
 If a field is specified in the `auto_populated_fields`, but does not meet the
 structural requirements, the client library generators **must not** enable
 automatic population for that field. Client library generators **may** emit an
 error during generation.
 
 Client libraries **must** reuse automatically populated values for retries of
-the same request. In other words, the automatically populated fields **must not**
-be regenerated for each RPC attempt with the same request message.
+the same request. In other words, the automatically populated fields
+**must not** be regenerated for each RPC attempt with the same request message.
 
 [AIP-155]: https://google.aip.dev/155