cloud_storage_clients: skip lexical_cast on empty strings

`parse_rest_error_response` method tries to read fields from xml response
and constructs `rest_error_response`. If a field is not found or is
empty then it defaults to empty string.

https://github.com/redpanda-data/redpanda/blob/d3c2f00c4071c2cbce1e1babdfc2291e3c9898ba/src/v/cloud_storage_clients/s3_client.cc#L411

Google Cloud Storage gives one of these replies which we parse but the
Error.Code path is not present in the response and we trip with a bad
lexical cast which results in an error log line.

With this commit we'll default to unknown error code in that case. We
already do the same for codes we don't recognize in the operator>>.
lexical_cast does not call the operator>> at all for empty strings.

Author: nvartolomei

src/v/cloud_storage_clients/s3_error.cc

@@ -420,7 +420,9 @@ rest_error_response::rest_error_response(
   std::string_view message,
   std::string_view request_id,
   std::string_view resource)
-  : _code(boost::lexical_cast<s3_error_code>(code))
+  : _code(
+      code.empty() ? s3_error_code::_unknown
+                   : boost::lexical_cast<s3_error_code>(code))
   , _code_str(code)
   , _message(message)
   , _request_id(request_id)