[processing] Fix Points to Paths terminates when an order by

nyalldawson · nyalldawson · commit b8c7305218b1 · 2018-03-16T17:09:32.000+11:00
field is empty (cherry-picked from 87e8432)
diff --git a/python/plugins/processing/algs/qgis/PointsToPaths.py b/python/plugins/processing/algs/qgis/PointsToPaths.py
@@ -162,7 +162,7 @@ def processAlgorithm(self, parameters, context, feedback):
             if feedback.isCanceled():
                 break
 
-            vertices.sort()
+            vertices.sort(key=lambda x: (x[0] is None, x[0]))
             f = QgsFeature()
             attributes = []
             if group_field_index >= 0:
