Server error when submitting trainee-match form without selected trainees
1. Go to the page for matching trainees to training sessions.
2. Select a training session, but do _not_ select a person.
3. Submit.
4. Boom - 500 error.

diff --git a/workshops/forms.py b/workshops/forms.py
index 28456ab8..3ed0fbc7 100644
--- a/workshops/forms.py
+++ b/workshops/forms.py
@@ -1410,7 +1410,7 @@ class BulkMatchTrainingRequestForm(forms.Form):
     def clean(self):
         super().clean()
 
-        if any(r.person is None for r in self.cleaned_data['requests']):
+        if any(r.person is None for r in self.cleaned_data.get('requests', [])):
             raise ValidationError('Some of the requests are not matched '
                                   'to a trainee yet. Before matching them to '
                                   'a training, you need to accept them '
['workshops/forms.py']