artifacts: allow missing tarball in BundleTastFiles

Now that recipes always attempts to upload artifacts, missing TastFiles
are less of an issue.

BUG=chromium:1086630
TEST=unit tests pass

Change-Id: I3ad0c8e33c426e4446ddc2dc92a22fd394a38519
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2241735
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
diff --git a/api/controller/artifacts.py b/api/controller/artifacts.py
index b4da262..ba191b0 100644
--- a/api/controller/artifacts.py
+++ b/api/controller/artifacts.py
@@ -246,12 +246,10 @@
 
   archive = artifacts.BundleTastFiles(chroot, sysroot, output_dir)
 
-  if archive is None:
-    cros_build_lib.Die(
-        'Could not bundle Tast files. '
-        'No Tast directories found for %s.', target)
-
-  output_proto.artifacts.add().path = archive
+  if archive:
+    output_proto.artifacts.add().path = archive
+  else:
+    logging.warning('Found no tast files for %s.', target)
 
 
 def _BundlePinnedGuestImagesResponse(input_proto, output_proto, _config):