Avoid a crash when share field is left empty

costs/models.py

@@ -256,7 +256,8 @@ class AbstractUse(models.Model):
 
     def clean(self):
         if hasattr(self, 'resource'):
-            usage = self.resource.used(except_by=self.service) + self.share
+            current_share = self.share or 0
+            usage = self.resource.used(except_by=self.service) + current_share
             if usage > self.resource.total_capacity:
                 raise ValidationError(
                     "Cannot use more than 100% of {})".format(self.resource))