Don't crash if ForeignKey field doesn't have keyword arguments

Fixes #230

Author: atodorov

pylint_django/transforms/foreignkey.py

@@ -42,7 +42,9 @@ def _get_model_class_defs_from_module(module, model_name, module_name):
 
 
 def infer_key_classes(node, context=None):
-    keyword_args = [kw.value for kw in node.keywords]
+    keyword_args = []
+    if node.keywords:
+        keyword_args = [kw.value for kw in node.keywords]
     all_args = chain(node.args, keyword_args)
 
     for arg in all_args: