grpc_field_extraction: fix empty ListValue indication (#35842)

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ProtobufWkt::Value is introduced by #35162

Commit Message:
Additional Description:
Risk Level: low
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:

Signed-off-by: Boteng Yao <boteng@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 77357cac6e6fd1dc4d91884a7cb93156f6e2feb7

envoy/extensions/filters/http/grpc_field_extraction/v3/config.proto

@@ -66,7 +66,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // 1. the extracted field names/values will be wrapped in be ``field<StringValue
 // or MapValue>`` -> ``values<ListValue of StringValue or StructValue>``, which will be added in the dynamic ``metadata<google.protobuf.Struct>``.
 //
-// 2. if the field value is empty, an empty ``<ListValue>`` will be set.
+// 2. if the field value is empty, an empty ``Value`` will be set.
 //
 // Performance
 // -----------