handle error when no links are found

This is to address https://github.com/tomchristie/django-rest-raml/issues/5

The problem is that if you try to generate RAML docs when you haven't set up any views, you get the above error (min called on an empty list).  

unfortunately, this PR is not very helpful since it doesn't actually surface a readable error to the user.  Not sure what the best way to address this would be...

rest_framework/schemas.py

@@ -263,6 +263,8 @@ class SchemaGenerator(object):
             view_endpoints.append((path, method, view))
 
         # Only generate the path prefix for paths that will be included
+        if not paths:
+            return None
         prefix = self.determine_path_prefix(paths)
 
         for path, method, view in view_endpoints: