Follow same default behavior whether we query for image or get it ourselves

Author: jepetty

src/azure-cli/azure/cli/command_modules/containerapp/custom.py

@@ -1575,7 +1575,7 @@ def start_containerappsjob(cmd,
         if image is not None:
             container_def["image"] = image if not is_registry_msi_system(registry_identity) else HELLO_WORLD_IMAGE
         else:
-            # Fetch the existing job definition to get the default image
+            Fetch the existing job definition to get the default image
             try:
                 containerappjob_def = ContainerAppsJobClient.show(cmd=cmd, resource_group_name=resource_group_name, name=name)
             except Exception as e:
@@ -1587,7 +1587,7 @@ def start_containerappsjob(cmd,
             existing_image = safe_get(containerappjob_def, "properties", "template", "containers", default=[{}])[0].get("image")
             if not existing_image:
                 raise ValidationError("Could not find an existing image for the containerapp job '{}'. Please specify --image.".format(name))
-            container_def["image"] = existing_image
+            container_def["image"] = existing_image if not is_registry_msi_system(registry_identity) else HELLO_WORLD_IMAGE
 
         if env_vars is not None:
             container_def["env"] = parse_env_var_flags(env_vars)