filter_record_modifier: fix handling empty msgpack
filter_record_modifier: fix handling empty msgpack

Signed-off-by: Takahiro YAMASHITA <8b9f03f78cb27351a99f79113f99e589b65190d5@gmail.com>

takahasi/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,fluent/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,fluent/fluent-bit,takahasi/fluent-bit,takahasi/fluent-bit,takahasi/fluent-bit,nokute78/fluent-bit,takahasi/fluent-bit,takahasi/fluent-bit,fluent/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit,nokute78/fluent-bit