bug#22027: 25.1.50; read-passwd fail to return default password

[Tino Calancha]

* lisp/subr.el (read-passwd):
read-passwd returns string consisting of (length password)
NULL characters, when:
*) first argument non-nil,
*) second argument string != "",
*) user accepts the default password.


emacs -Q:

(read-passwd "pass: " t "my-pass") RET RET
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