🐛(backend) manage invitation partial update without email

An invitation can be updated to change its role. The front use a PATCH
sending only the changed role, so the email is missing in the
InivtationSerializer.validate method. We have to check first if an email
is present before working on it.

src/backend/core/api/serializers.py

@@ -749,7 +749,8 @@ class InvitationSerializer(serializers.ModelSerializer):
         if self.instance is None:
             attrs["issuer"] = user
 
-        attrs["email"] = attrs["email"].lower()
+        if attrs.get("email"):
+            attrs["email"] = attrs["email"].lower()
 
         return attrs