Fix calc displayname and add tests

lib/src/Event.dart

@@ -137,6 +137,8 @@ class Event {
         print("jsonObj decode of event content failed: ${e.toString()}");
         content = {};
       }
+    else
+      content = {};
 
     return Event(
       jsonObj["event_id"] ?? jsonObj["id"],

lib/src/User.dart

@@ -72,7 +72,7 @@ class User {
 
   /// Returns the displayname or the local part of the Matrix ID if the user
   /// has no displayname.
-  String calcDisplayname() => displayName.isEmpty
+  String calcDisplayname() => (displayName == null || displayName.isEmpty)
       ? id.replaceFirst("@", "").split(":")[0]
       : displayName;
 

test/User_test.dart

@@ -51,5 +51,14 @@ void main() {
       expect(user.powerLevel, powerLevel);
       expect(user.calcDisplayname(), displayName);
     });
+
+    test("calcDisplayname", () async {
+      final User user1 = User("@alice:example.com");
+      final User user2 = User("@alice:example.com", displayName: "SuperAlice");
+      final User user3 = User("@alice:example.com", displayName: "");
+      expect(user1.calcDisplayname(), "alice");
+      expect(user2.calcDisplayname(), "SuperAlice");
+      expect(user3.calcDisplayname(), "alice");
+    });
   });
 }