Fix ScheduledActions throwing an exception if no weedays have been set

Fixes an issue introduced in commit 8087978. We assumed at least one
weekday was always set in the recurrence of weekly actions. The UI
dialog enforces it. However, GnuCash desktop doesn't. So we must check
it for the case of imported scheduled actions.

app/src/main/java/org/gnucash/android/model/ScheduledAction.java

@@ -241,12 +241,19 @@ private long computeNextScheduledExecutionTimeStartingAt(long startTime) {
      * Computes the next time that this weekly scheduled action is supposed to be
      * executed starting at startTime.
      *
+     * If no weekdays have been set (GnuCash desktop allows it), it will return a
+     * date in the future to ensure ScheduledActionService doesn't execute it.
+     *
      * @param startTime LocalDateTime to use as start to compute the next schedule.
      *
-     * @return Next run time as a LocalDateTime
+     * @return Next run time as a LocalDateTime. A date in the future, if no weekdays
+     *      were set in the Recurrence.
      */
     @NonNull
     private LocalDateTime computeNextWeeklyExecutionStartingAt(LocalDateTime startTime) {
+        if (mRecurrence.getByDays().isEmpty())
+            return LocalDateTime.now().plusDays(1); // Just a date in the future
+
         // Look into the week of startTime for another scheduled weekday
         for (int weekDay : mRecurrence.getByDays() ) {
             int jodaWeekDay = convertCalendarWeekdayToJoda(weekDay);

app/src/test/java/org/gnucash/android/test/unit/model/ScheduledActionTest.java

@@ -20,6 +20,7 @@
 import org.gnucash.android.model.Recurrence;
 import org.gnucash.android.model.ScheduledAction;
 import org.joda.time.DateTime;
+import org.joda.time.LocalDateTime;
 import org.junit.Test;
 
 import java.sql.Timestamp;
@@ -176,6 +177,25 @@ public void weeklyActionsWithMultiplier_shouldBeDueOnTheWeekdaySet() {
                 .isEqualTo(expectedNextDueDate);
     }
 
+    /**
+     * Weekly actions should return a date in the future when no
+     * weekdays have been set in the recurrence.
+     *
+     * See ScheduledAction.computeNextTimeBasedScheduledExecutionTime()
+     */
+    @Test
+    public void weeklyActionsWithoutWeekdaySet_shouldReturnDateInTheFuture() {
+        ScheduledAction scheduledAction = new ScheduledAction(ScheduledAction.ActionType.BACKUP);
+        Recurrence recurrence = new Recurrence(PeriodType.WEEK);
+        recurrence.setByDays(Collections.<Integer>emptyList());
+        scheduledAction.setRecurrence(recurrence);
+        scheduledAction.setStartTime(new DateTime(2016, 6, 6, 9, 0).getMillis());
+        scheduledAction.setLastRun(new DateTime(2017, 4, 12, 9, 0).getMillis());
+
+        long now = LocalDateTime.now().toDate().getTime();
+        assertThat(scheduledAction.computeNextTimeBasedScheduledExecutionTime()).isGreaterThan(now);
+    }
+
     private long getTimeInMillis(int year, int month, int day) {
         Calendar calendar = Calendar.getInstance();
         calendar.set(year, month, day);