Fix case no interval could be found

lib/variants_simulator.py

@@ -181,6 +181,8 @@ class VariantsSimulator:
             my_start = min(my_stretch[1], end)
         if my_start < end:
             intervals.append([my_start, end])
+        if len(intervals) == 0:  # No valid interval found
+            return 0, 0
         intervals.sort(key=lambda x: x[0]-x[1])
         return intervals[0]