Fixing mappings with and without contexts

Returning default mapping if contexts are defined but none applies

src/rules.py

@@ -114,7 +114,7 @@ def get_mappings_for(self, literal, context_map):
                 return self._mappings[literal]['context'][context]
 
         # Nothing matches
-        return None
+        return self._mappings[literal]['default']
 
 class RuleMaker(object):
     def __init__(self, configuration, dataset, output_file_name):