Update coderdojochi/models/session.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: karbassi

coderdojochi/models/session.py

@@ -275,6 +275,8 @@ def save(self, *args, **kwargs):
         self.mentor_capacity = max(self.mentor_capacity, 0)
 
         # Capacity check
+        if self.capacity is None:
+            self.capacity = 0
         self.capacity = max(self.capacity, 0)
 
         super(Session, self).save(*args, **kwargs)