date: Validate ISO 8601 duration strings for null bytes early

[adapik]

timelib takes a NUL-terminated string, so everything after an embedded NUL byte was silently ignored. Now it throws the same Time\TimeException as for any other malformed string.

// Before: Time\Duration of 5 seconds
// Now: Time\TimeException: The ISO 8601 duration string could not be parsed
Time\Duration::fromIso8601DurationString("PT5S\0garbage");