Merge pull request #6957 from bashtage/glm-null

BUG: Fix check for offset_exposure in null

statsmodels/genmod/generalized_linear_model.py

@@ -1635,7 +1635,7 @@ def null(self):
 
         kwargs = model._get_init_kwds()
         kwargs.pop('family')
-        if hasattr(self, '_offset_exposure'):
+        if hasattr(self.model, '_offset_exposure'):
             return GLM(endog, exog, family=self.family,
                        **kwargs).fit().fittedvalues
         else: