MXS-1731: Ignore empty values in persisted configs

If a MaxScale-generated configuration defines an empty value, it is
ignored with the assumption that the next modification will cause the
problem to correct itself.

maxscale-system-test/nodes.h

@@ -132,7 +132,7 @@ public:
      * @param i Node index
      * @return exit code of the system command or 1 in case of i > N
      */
-    int copy_to_node_legacy(const char* src, const char* dest, int i);
+    int copy_to_node_legacy(const char* src, const char* dest, int i = 0);
     int copy_to_node(int i, const char* src, const char* dest);
 
     /**