Permit empty passphrases when creating HW wallet

They used to be confused as a user cancel.
Fixes #1788
Also fix Cancel pressed in passphrase dialog when *restoring*
a hardware wallet in install wizard; it used to be taken as an
empty passphrase.  Like the password dialog it now cancels the
wizard.

gui/qt/password_dialog.py

@@ -146,7 +146,11 @@ class PasswordLayout(object):
         return None
 
     def new_password(self):
-        return unicode(self.new_pw.text()) or None
+        pw = unicode(self.new_pw.text())
+        # Empty passphrases are fine and returned empty.
+        if pw == "" and self.kind != PW_PASSPHRASE:
+            pw = None
+        return pw
 
 
 class PasswordDialog(WindowModalDialog):