order_by.cc: Use IsKeyFieldPath() instead of == FieldPath::KeyFieldPath() (#9550)

Author: dconeybe

Firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+- [changed] Potentially fixed a crash during application exit caused by an
+  assertion about ordering documents by missing fields (#9258).
 - [changed] Add more details to the assertion failure in Query::Comparator() to
   help with future debugging (#9258).
 

Firestore/core/src/core/order_by.cc

@@ -67,7 +67,7 @@ void AssertBothOptionalsHaveValues(
 ComparisonResult OrderBy::Compare(const Document& lhs,
                                   const Document& rhs) const {
   ComparisonResult result;
-  if (field_ == FieldPath::KeyFieldPath()) {
+  if (field_.IsKeyFieldPath()) {
     result = lhs->key().CompareTo(rhs->key());
   } else {
     absl::optional<google_firestore_v1_Value> value1 = lhs->field(field_);