grit: Add fallback_to_default_layout attribute to <output>

grit/node/include.py

Index: grit/node/include.py
diff --git a/grit/node/include.py b/grit/node/include.py
index 8d32064095ce885061dd3d3ca9b28680c3afd949..ebcd53108f503005dadae9ae67b192245fb0a66d 100644
--- a/grit/node/include.py
+++ b/grit/node/include.py
@@ -68,7 +68,11 @@ class IncludeNode(base.Node):
     """Returns the file for the specified language.  This allows us to return
     different files for different language variants of the include file.
     """
-    return self.ToRealPath(self.GetInputPath())
+    input_path = self.GetInputPath()
+    if input_path is None:
+      return None
+
+    return self.ToRealPath(input_path)
 
   def GetDataPackPair(self, lang, encoding):
     """Returns a (id, string) pair that represents the resource id and raw