Update devicelab build+test artifact check

Change-Id: I2e5ac178c48e43455490bf01e76a54a1d7241250
Bug: https://github.com/flutter/flutter/issues/103542
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/33067
Reviewed-by: Casey Hillers <chillers@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>

recipes/devicelab/devicelab_drone_build_test.py

diff --git a/recipes/devicelab/devicelab_drone_build_test.py b/recipes/devicelab/devicelab_drone_build_test.py
index 334c557..d7790cd 100644
--- a/recipes/devicelab/devicelab_drone_build_test.py
+++ b/recipes/devicelab/devicelab_drone_build_test.py
@@ -45,7 +45,7 @@
   step_result = api.gsutil.list(
       url, stdout=api.raw_io.output_text(), ok_ret='any'
   ).stdout.rstrip()
-  if step_result == url:
+  if step_result:
     artifacts_exist = True
   else:
     artifacts_exist = False