Fix asset_utils when no service_account_json

Change-Id: I0524c96e3a6ab1a342872d5d0d903e87526dffeb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256230
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
diff --git a/infra/bots/assets/asset_utils.py b/infra/bots/assets/asset_utils.py
index 834e696..9dd9f45 100755
--- a/infra/bots/assets/asset_utils.py
+++ b/infra/bots/assets/asset_utils.py
@@ -52,6 +52,8 @@
     self._cipd_url = cipd_url
     if service_account_json:
       self._service_account_json = os.path.abspath(service_account_json)
+    else:
+      self._service_account_json = None
     self._check_setup()
 
   def _check_setup(self):