The text does not correspond to what you test for. It returns the empty string, not the empty sequence.

The value is returned from regex-group(n) in pf:tokenize-uri.
regex-group: "The function returns the zero-length string if there is no captured substring with the relevant number."

It seems you can test for an empty sequence if you want to by simply saying select="()" in the test, but pf:get-scheme declares its return value as xs:string anyway so I assume that it's just the test label that is incorrect.