fix(AIP-160): clarify unset field in traversal behavior (#1469)

* fix(AIP-160): clarify unset field in traversal behavior

* chore: include nuance of != in unset case

Author: noahdietz

aip/general/0160.md

@@ -138,6 +138,13 @@ the service **should** return an error with `INVALID_ARGUMENT`. A service
 **may** permit traversal to undefined keys on maps and structs, and **should**
 document how it behaves in this situation.
 
+When evaluating an expression involving a traversal, if any non-primitive field
+in the chain is not set on the entry being evaluated, the entry **should** be
+skipped i.e. not match the filter expression. This applies even when the
+comparison is a `!=`, which would imply matching on empty values. In the
+examples above, if resource field `a` is not set on the resource instance, that
+instance is skipped as a non-match.
+
 **Important:** The `.` operator **must not** be used to traverse through a
 repeated field or list, except for specific use with the `:` operator.
 
@@ -224,4 +231,5 @@ Schematic validation refers, but is not limited to, the following:
 
 ## Changelog
 
+- **2025-01-07**: Clarify behavior for unset field in traversal operator chain.
 - **2024-12-11**: Move non-compliant filter guidance to Validation section.