String optionalOr(@Nonnull String str, String defaults) {
        return str.isEmpty() ? defaults : str;
    }
Check if a given non-null String instance is "missing" according to ProtoBuf's missing field
rules. If the String is missing, the given default value will be returned. Otherwise, the
string itself will be returned.

@param str the input String
@param defaults the default value for the string
@return 'str' if it is not empty according to ProtoBuf rules; 'defaults' otherwise
grpc-server/src/main/java/com/netflix/conductor/grpc/server/service/GRPCHelper.java