api: controller/packages: proto field default value fallback

Add a fallback to an empty string for the firmware versions since
a version can come up empty on variants, but the object evaluates
to true when either is set.

BUG=b:240329712
TEST=CQ

Change-Id: I592c85fabefb529fc1f8deababbc0c0fbda20e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3789780
Reviewed-by: Lizzy Presland <zland@google.com>
Auto-Submit: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/packages.py b/api/controller/packages.py
index f9d649b..45eb905 100644
--- a/api/controller/packages.py
+++ b/api/controller/packages.py
@@ -223,9 +223,9 @@
       build_target.name)
 
   fw_versions = packages.determine_firmware_versions(build_target)
-  if fw_versions:
-    build_target_metadata.main_firmware_version = fw_versions.main_fw_version
-    build_target_metadata.ec_firmware_version = fw_versions.ec_fw_version
+  build_target_metadata.main_firmware_version = (
+      fw_versions.main_fw_version or '')
+  build_target_metadata.ec_firmware_version = fw_versions.ec_fw_version or ''
 
   build_target_metadata.kernel_version = packages.determine_kernel_version(
       build_target) or ''