VOL-501 Avoid assigning a None store_id when node does not have valid store_id yet

Change-Id: I0ca7ca09293279567c34dfc5cb8275f51da2d690
diff --git a/voltha/worker.py b/voltha/worker.py
index b47accc..154c931 100644
--- a/voltha/worker.py
+++ b/voltha/worker.py
@@ -118,7 +118,7 @@
                 core_stores = [c['Value'] for c in results if
                                c['Key'] == self.coord.assignment_prefix +
                                self.instance_id + '/' +
-                               self.coord.core_storage_suffix]
+                               self.coord.core_storage_suffix and c['Value']]
                 if core_stores:
                     self.mycore_store_id = core_stores[0]
                     log.debug('store-assigned',