Lost microseconds precision when parsing `00.000000`

[andrhua]

Steps to reproduce

> format = "{YYYY}-{0M}-{0D} {h24}:{m}:{s}{ss} {Zname}"
> Timex.parse!("2022-06-24 08:00:00.000000 America/New_York", format).microsecond
{0, 0}

Description of issue

Expected to get {0, 6}