[F&O] string-length

The signature for string-length is

   fn:string-length($arg as xs:string) as xs:integer

but the description says:

"If the value of $arg is the empty sequence, the xs:integer 0 is returned".

It seems to me that the signature should be changed to be

   fn:string-length($arg as xs:string?) as xs:integer

so that an empty sequence can be recognized as a valid argument

Alberto