[Batch] Fix Azure#23445: az batch pool supported-images list: Fix t…

…he `NoneType object has no attribute startswith` bug for getting supported images list (Azure#23449)

src/azure-cli/azure/cli/command_modules/batch/_validators.py

@@ -304,10 +304,10 @@ def validate_client_parameters(cmd, namespace):
         namespace.account_endpoint = cmd.cli_ctx.config.get('batch', 'endpoint', None)
 
     # Simple validation for account_endpoint
-    if not (namespace.account_endpoint.startswith('https://') or
-            namespace.account_endpoint.startswith('http://')):
+    if namespace.account_endpoint and not (namespace.account_endpoint.startswith('https://') or
+                                           namespace.account_endpoint.startswith('http://')):
         namespace.account_endpoint = 'https://' + namespace.account_endpoint
-    namespace.account_endpoint = namespace.account_endpoint.rstrip('/')
+        namespace.account_endpoint = namespace.account_endpoint.rstrip('/')
     # if account name is specified but no key, attempt to query if we use shared key auth
     if namespace.account_name and namespace.account_endpoint and not namespace.account_key:
         if cmd.cli_ctx.config.get('batch', 'auth_mode', 'shared_key') == 'shared_key':