[perl #81016] Document ord("")

[perl #81016] Document ord("")

The following patch clarifies what ord("") does in pod/perlfunc.pod.

From the current documentation, it's not documented what it does, and one
could guess several different things: returns zero, returns undef, returns
zero with a warning, returns undef with a warning, dies.  (There's precedent
for dying: some BASIC implementations give an error if you ask for ASC("")
-- here ASC is their equivalent of our ord function.)