Do not save empty choice parameters for non-choice tasks

python/educational-core/student/src/com/jetbrains/edu/learning/courseFormat/Task.java

@@ -44,7 +44,7 @@ public class Task implements StudyItem {
   @Transient private Lesson myLesson;
   @Expose @SerializedName("update_date") private Date myUpdateDate;
 
-  @Expose @SerializedName("choice_parameters") private ChoiceParameters myChoiceParameters = new ChoiceParameters();
+  @Expose @SerializedName("choice_parameters") private ChoiceParameters myChoiceParameters;
   private int myActiveSubtaskIndex = 0;
   @SerializedName("last_subtask_index")
   @Expose private int myLastSubtaskIndex = 0;
@@ -54,6 +54,12 @@ public class Task implements StudyItem {
   public Task(@NotNull final String name) {
     this.name = name;
   }
+  
+  public static Task createChoiceTask(@NotNull String name) {
+    final Task task = new Task(name);
+    task.setChoiceParameters(new ChoiceParameters());
+    return task;
+  }
 
   /**
    * Initializes state of task file
@@ -326,7 +332,7 @@ public class Task implements StudyItem {
   }
   
   public boolean isChoiceTask() {
-    return !myChoiceParameters.getChoiceVariants().isEmpty();
+    return myChoiceParameters != null;
   }
 
   // used for serialization

python/educational-core/student/src/com/jetbrains/edu/learning/stepic/EduAdaptiveStepicConnector.java

@@ -156,7 +156,7 @@ public class EduAdaptiveStepicConnector {
                                             @NotNull String lessonName,
                                             @NotNull StepicWrappers.Step block,
                                             int stepId) {
-    final Task task = new Task(lessonName);
+    final Task task = Task.createChoiceTask(lessonName);
     task.setStepId(stepId);
     task.setText(block.text);