fix bug when submitting some empty fields in visit admin form

visits/admin.py

@@ -16,7 +16,7 @@ class RegistrationsOpenFilter(admin.SimpleListFilter):
     https://docs.djangoproject.com/fr/2.0/ref/contrib/admin/#django.contrib.admin.ModelAdmin.list_filter
     """
 
-    title = "état des inscriptions"
+    title = 'état des inscriptions'
     parameter_name = 'registrations_open'
 
     def lookups(self, request, model_admin):
@@ -47,18 +47,22 @@ class VisitForm(forms.ModelForm):
 
         - Deadline must be before the date
         - End time must be after start time
+
+        Keep in mind that values may be `None` if not provided in the form.
         """
         cleaned_data = super().clean()
         date = cleaned_data.get('date')
         deadline = cleaned_data.get('deadline')
         start_time = cleaned_data.get('start_time')
         end_time = cleaned_data.get('end_time')
+        if deadline is not None:
             if deadline.date() >= date:
                 error = forms.ValidationError(
                     "La date limite d'inscription doit être avant la "
                     "date de la sortie."
                 )
                 self.add_error('deadline', error)
+        if end_time is not None and start_time is not None:
             if end_time <= start_time:
                 error = forms.ValidationError(
                     "L'heure de fin doit être après l'heure de début.")