Skip to call metadata store API if no data available

Signed-off-by: Alan Zhang <shuai.xyz@gmail.com>

Author: alnzng

samza-core/src/main/java/org/apache/samza/container/grouper/task/TaskAssignmentManager.java

@@ -142,11 +142,18 @@ public void writeTaskContainerMappings(Map<String, Map<String, TaskMode>> mappin
         }
       }
     }
-
-    taskContainerMappingMetadataStore.deleteAll(taskContainerMappingsToBeDeleted);
-    taskModeMappingMetadataStore.deleteAll(taskModeMappingsToBeDeleted);
-    taskContainerMappingMetadataStore.putAll(taskContainerMappingsToBeStored);
-    taskModeMappingMetadataStore.putAll(taskModeMappingsToBeStored);
+    if (!taskContainerMappingsToBeDeleted.isEmpty()) {
+      taskContainerMappingMetadataStore.deleteAll(taskContainerMappingsToBeDeleted);
+    }
+    if (!taskModeMappingsToBeDeleted.isEmpty()) {
+      taskModeMappingMetadataStore.deleteAll(taskModeMappingsToBeDeleted);
+    }
+    if (!taskContainerMappingsToBeStored.isEmpty()) {
+      taskContainerMappingMetadataStore.putAll(taskContainerMappingsToBeStored);
+    }
+    if (!taskModeMappingsToBeStored.isEmpty()) {
+      taskModeMappingMetadataStore.putAll(taskModeMappingsToBeStored);
+    }
   }
 
   /**