Fix crash when processLinearMove gets called on something else than G0 or G1

Return the original line then.

plugins/PostProcessingPlugin/scripts/ChangeAtZ.py

@@ -1101,7 +1101,7 @@ class ChangeAtZProcessor:
 
         # if it's not a linear move, we don't care
         if linear_command is None:
-            return
+            return line
 
         # get our linear move parameters
         feed_rate = linear_command.Arguments["F"]