controller: don't die on no firmware artifacts

If we don't find any firmware builds to upload, warn and return nothing
rather than aborting the whole build.

BUG=b:195415535
TEST=./run_tests.py

Change-Id: I9f26ed5405e62fd83c078345861dbd444f19fcda
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3290886
Tested-by: George Engelbrecht <engeg@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jack Neus <jackneus@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: George Engelbrecht <engeg@google.com>
diff --git a/api/controller/artifacts.py b/api/controller/artifacts.py
index 2a3aaba..5586fea 100644
--- a/api/controller/artifacts.py
+++ b/api/controller/artifacts.py
@@ -459,9 +459,10 @@
   archive = artifacts.BuildFirmwareArchive(chroot, sysroot, output_dir)
 
   if archive is None:
-    cros_build_lib.Die(
+    logging.warning(
         'Could not create firmware archive. No firmware found for %s.',
         sysroot_path)
+    return
 
   output_proto.artifacts.add().path = archive