ota_utils: Don't fail if metadata proto doesn't exist

The protobuf addition is present only in S+
while the legacy metadata is always present

Test: tools/gen_update_config.py with ota from R
Change-Id: I482c75b6b3b2b1ade139e59dc9dc57dbf9731d45
diff --git a/tools/releasetools/ota_utils.py b/tools/releasetools/ota_utils.py
index 5737009..4833d7b 100644
--- a/tools/releasetools/ota_utils.py
+++ b/tools/releasetools/ota_utils.py
@@ -569,7 +569,8 @@
       tokens.append('metadata.pb:' + ' ' * 15)
     else:
       tokens.append(ComputeEntryOffsetSize(METADATA_NAME))
-      tokens.append(ComputeEntryOffsetSize(METADATA_PROTO_NAME))
+      if METADATA_PROTO_NAME in zip_file.namelist():
+          tokens.append(ComputeEntryOffsetSize(METADATA_PROTO_NAME))
 
     return ','.join(tokens)