Add empty user at import, ignore them for learning progress

src/main/java/de/thm/arsnova/dao/CouchDBDao.java

@@ -1772,6 +1772,8 @@ public class CouchDBDao implements IDatabaseDao {
 					answerDoc.put("timestamp", a.getTimestamp());
 					answerDoc.put("piRound", a.getPiRound());
 					answerDoc.put("abstention", a.isAbstention());
+					// we do not store the user's name
+					answerDoc.put("user", "");
 					answers.add(answerDoc);
 				}
 			}

src/main/java/de/thm/arsnova/domain/CourseScore.java

@@ -35,6 +35,7 @@ public class CourseScore implements Iterable<QuestionScore> {
 
 	/**
 	 * @pre questionId has been added before.
+	 * @pre username is set
 	 * @param questionId
 	 * @param username
 	 * @param userscore
@@ -45,6 +46,10 @@ public class CourseScore implements Iterable<QuestionScore> {
 			// Most likely this is a question that has no learning progress value.
 			return;
 		}
+		if (username == null || username.isEmpty()) {
+			// Precondition failed: ignore anonymous users
+			return;
+		}
 		QuestionScore questionScore = scores.get(questionId);
 		questionScore.add(username, userscore);
 	}