The ICU library does not provide a way to pass 'no timezone' to the SimpleDateFormat/Calendar objects. We need a way to specify 'no timezone' to be compliant with the dfdl:timezoneFromDateTime, and the like, functions.

Essentially, the function will never be able to return an 'empty string' when there is no timezone.

From spec:

"Returns the timezone component of $arg if
any as an xs:string. The $arg is of type
xs:dateTime, xs:date and xs:time respectively.

If $arg has a timezone component, then the
result is a string in the format of an ISO Time
zone designator. Interpreted as an offset from
UTC, its value may range from +14:00 to -
14:00 hours, both inclusive. The UTC time
zone is represented as "+00:00". If the $arg
has no timezone component, then "" (empty
string) is returned."