ORA-FO-EMPTY-RETURN 2.3 fn:string

*Functions and Operators, Section 2.3*
Inconsistencies in function return values.

When the input to a function is "empty" or "missing", some functions 
return the empty sequence, some functions return false, some return the 
empty string - and the rationale seems arbitrary.

Example: fn:string returns a xs:string if the input is the empty 
sequence. So does fn:namespace-uri. But fn:base-uri returns the empty 
sequence, rather than an empty string, if the node doesn't define a 
base-uri.