[cmake,bugfix] Handle case where module search path is not in list.

With new modules there were cases where cmake/modules was not in the
modules path which made configuration fail. This patch fixes this case.

cmake/modules/DuneMacros.cmake

@@ -360,7 +360,9 @@ macro(dune_create_dependency_tree)
 	if(start EQUAL ${length})
 	  list(APPEND CMAKE_MODULE_PATH ${_my_path})
 	else(start EQUAL ${length})
-	  list(INSERT CMAKE_MODULE_PATH ${start} ${_my_path})
+	  if(_my_path)
+	    list(INSERT CMAKE_MODULE_PATH ${start} ${_my_path})
+	  endif(_my_path)
 	endif(start EQUAL ${length})
       endif(start EQUAL -1)
     endif(length GREATER 0)