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/installwizard.py

@@ -208,13 +208,15 @@ class InstallWizard(QDialog, MessageBoxMixin, WizardBase):
         self.set_main_layout(playout.layout())
         return playout.new_password()
 
-    def request_passphrase(self, device_text, restore=True):
-        """Request a passphrase for a wallet from the given device and
-        confirm it.  restore is True if restoring a wallet.  Should return
+    def request_passphrase(self, device_text):
+        """When restoring a wallet, request the passphrase that was used for
+        the wallet on the given device and confirm it.  Should return
         a unicode string."""
-        if restore:
-            msg = MSG_RESTORE_PASSPHRASE % device_text
-        return unicode(self.pw_layout(msg, PW_PASSPHRASE) or '')
+        phrase = self.pw_layout(MSG_RESTORE_PASSPHRASE % device_text,
+                                PW_PASSPHRASE)
+        if phrase is None:
+            raise UserCancelled
+        return phrase
 
     def request_password(self, msg=None):
         """Request the user enter a new password and confirm it.  Return

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):