[cmake,bugfix] Allow usage of empty CMAKE_MODULE_PATH.

At least for dune-common the CMAKE_MODULE_PATH may be empty
and removing an item from an empty list is an error. This
patch fixes this issue by checking whether th list is empty.

cmake/modules/DuneMacros.cmake

@@ -274,7 +274,9 @@ macro(dune_create_dependency_leafs depends depends_versions suggests suggests_ve
 endmacro(dune_create_dependency_leafs)
 
 macro(dune_create_dependency_tree)
-  list(REMOVE_ITEM CMAKE_MODULE_PATH ${dune-common_MODULE_PATH})
+  if(${dune-common_MODULE_PATH})
+    list(REMOVE_ITEM CMAKE_MODULE_PATH ${dune-common_MODULE_PATH})
+  endif(${dune-common_MODULE_PATH})
   list(FIND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules start)
   set(ALL_DEPENDENCIES "")
   if(DEPENDS_MODULE OR SUGGESTS_MODULE)