fix: Get direct chat from user ID method

The method was not type safe and therefore there
was no warning that with the sortOrder changes
now DateTimes are compared which leads to
an exception in the app if not using converting to milliseconds first.

lib/src/client.dart

@@ -306,8 +306,8 @@ class Client extends MatrixApi {
                 null,
                 (prev, r) => prev == null
                     ? r
-                    : (prev.lastEvent.originServerTs <
-                            r.lastEvent.originServerTs
+                    : (prev.lastEvent.originServerTs.millisecondsSinceEpoch <
+                            r.lastEvent.originServerTs.millisecondsSinceEpoch
                         ? r
                         : prev))
             .id;