Only load domain at start if there are executions

This prevent slow startup that may occur
from the https://api.dns.public.aleph.sh/instances/list

Author: olethanh

src/aleph/vm/pool.py

@@ -332,7 +332,8 @@ async def load_persistent_executions(self):
             else:
                 execution.uuid = saved_execution.uuid
                 await execution.record_usage()
-        await self.update_domain_mapping(force_update=True)
+        if self.executions:
+            await self.update_domain_mapping(force_update=True)
         logger.info(f"Loaded {len(self.executions)} executions")
 
     async def stop(self):