api [nfc]: Implement empty() on GetMessagesResult

Author: chrisbobbe

lib/api/route/messages.dart

@@ -150,6 +150,17 @@ class GetMessagesResult {
     required this.messages,
   });
 
+  /// An example empty result, for use in tests.
+  GetMessagesResult.empty() :
+    // this `anchor` was observed empirically;
+    // also, it's not representative of all requests.
+    anchor = 10000000000000000,
+    foundNewest = true,
+    foundOldest = true,
+    foundAnchor = false,
+    historyLimited = false, // not representative of all requests
+    messages = [];
+
   factory GetMessagesResult.fromJson(Map<String, dynamic> json) =>
       _$GetMessagesResultFromJson(json);