api: packages.py: Proto field None guards.

Add more guards again setting proto fields to None. Add tests
for the firmware versions fields.

BUG=b:244265121
TEST=run_tests

Change-Id: I112edcafd06649f43a4d512ef36f92641711259b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3866297
Tested-by: Alex Klein <saklein@chromium.org>
Auto-Submit: Alex Klein <saklein@chromium.org>
Reviewed-by: Sergey Frolov <sfrolov@google.com>
Commit-Queue: Jim Pollock <jmpollock@chromium.org>
diff --git a/api/controller/packages.py b/api/controller/packages.py
index 45eb905..48f3084 100644
--- a/api/controller/packages.py
+++ b/api/controller/packages.py
@@ -245,10 +245,10 @@
         key_id = packages.get_key_id(build_target, model)
         model_metadata = output_proto.model_metadata.add()
         model_metadata.model_name = model
-        model_metadata.ec_firmware_version = ec
+        model_metadata.ec_firmware_version = ec or ''
         model_metadata.firmware_key_id = key_id
-        model_metadata.main_readonly_firmware_version = main_ro
-        model_metadata.main_readwrite_firmware_version = main_rw
+        model_metadata.main_readonly_firmware_version = main_ro or ''
+        model_metadata.main_readwrite_firmware_version = main_rw or ''
 
 
 def _HasPrebuiltSuccess(_input_proto, output_proto, _config):