Check if next page token is empty string

https://issuetracker.google.com/issues/133254108

app/build.gradle

@@ -21,8 +21,8 @@ android {
     defaultConfig {
         testApplicationId "org.tasks.test"
         applicationId "org.tasks"
-        versionCode 579
-        versionName "6.6.3"
+        versionCode 580
+        versionName "6.6.4"
         targetSdkVersion 28
         minSdkVersion 16
         multiDexEnabled true

app/src/main/java/org/tasks/gtasks/GoogleTaskSynchronizer.java

@@ -346,7 +346,7 @@ public class GoogleTaskSynchronizer {
           tasks.addAll(items);
         }
         nextPageToken = taskList.getNextPageToken();
-      } while (nextPageToken != null);
+      } while (!Strings.isNullOrEmpty(nextPageToken));
 
       for (com.google.api.services.tasks.model.Task gtask : tasks) {
         String remoteId = gtask.getId();