api/controller/packages: None field assignment

Add a fallback to the default value for the field to avoid assigning
None to the field.

BUG=b:231716807
TEST=CQ

Change-Id: Ib2ee519e5eac853f69f19bd1a27a2ef3e8a7d41f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3636801
Reviewed-by: Benjamin Shai <bshai@google.com>
Tested-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Auto-Submit: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/packages.py b/api/controller/packages.py
index a9f98ae..f9d649b 100644
--- a/api/controller/packages.py
+++ b/api/controller/packages.py
@@ -228,7 +228,7 @@
     build_target_metadata.ec_firmware_version = fw_versions.ec_fw_version
 
   build_target_metadata.kernel_version = packages.determine_kernel_version(
-      build_target)
+      build_target) or ''
   fingerprints = packages.find_fingerprints(build_target)
   build_target_metadata.fingerprints.extend(fingerprints)