avoid not valid local variable in certain conditions

Author: aliel

src/aleph/sdk/domain.py

@@ -92,6 +92,7 @@ def get_required_dns_rules(self, url, target, owner: Optional[str] = None):
         target = target.lower()
         dns_rules = []
 
+        cname_value = None
         if target == "ipfs":
             cname_value = settings.DNS_IPFS_DOMAIN
         elif target == "program":