Propose to allow empty since this function operates like a casting/constructor 
and should accept empty (as the constructor functions do):

fn:dateTime($arg1 as xs:date?, $arg2 as xs:time?) as xs:dateTime?
The result xs:dateTime has a date component whose value is equal to $arg1 and 
a time component whose value is equal to $arg2. If any of the arguments is the 
empty sequence, the result is the empty sequence.