JITLib: NodeProxy - remove inconsistent nodeMap_() method

nodeMap_() is inconsitent with `var <nodeMap` and causes an
Error when called with `nil` which needs a Library recompilation.

Signed-off-by: Yvan Volochine <yvan.volochine@gmail.com>
JITLib: NodeProxy - add a warning when calling `setNodeMap()` with `nil`

- calling `setNodeMap(nil)` will cause an Error which forces to recompile
  the Library. Instead, print a warning and return.
- Error stack: https://gist.github.com/gusano/16ccbb6306ff2903d607

Signed-off-by: Yvan Volochine <yvan.volochine@gmail.com>