qml: explicitly use None when empty string is used as password

backend requires None, Qt5 passes empty string

electrum/gui/qml/qedaemon.py

@@ -171,6 +171,10 @@ class QEDaemon(AuthMixin, QObject):
 
         self._logger.debug('load wallet ' + str(self._path))
 
+        # map empty string password to None
+        if password == '':
+            password = None
+
         if not password:
             password = self._password
 
@@ -225,7 +229,7 @@ class QEDaemon(AuthMixin, QObject):
         wallet = self.daemon._wallets[self._path]
         self._current_wallet = QEWallet.getInstanceFor(wallet)
         self.availableWallets.updateWallet(self._path)
-        self._current_wallet.password = password
+        self._current_wallet.password = password if password else None
         self.walletLoaded.emit(self._name, self._path)
 
 
@@ -312,6 +316,9 @@ class QEDaemon(AuthMixin, QObject):
     @pyqtSlot(str)
     def setPassword(self, password):
         assert self._use_single_password
+        # map empty string password to None
+        if password == '':
+            password = None
         self._logger.debug('about to set password for ALL wallets')
         self.daemon.update_password_for_directory(old_password=self._password, new_password=password)
         self._password = password

electrum/gui/qml/qewallet.py

@@ -710,14 +710,20 @@ class QEWallet(AuthMixin, QObject, QtEventListener):
 
     @pyqtSlot(str)
     def set_password(self, password):
+        if password == '':
+            password = None
+
         storage = self.wallet.storage
 
         # HW wallet not supported yet
         if storage.is_encrypted_with_hw_device():
             return
 
+        current_password = self.password if self.password != '' else None
+
         try:
-            self.wallet.update_password(self.password, password, encrypt_storage=True)
+            self._logger.info(f'PW change from {current_password} to {password}')
+            self.wallet.update_password(current_password, password, encrypt_storage=True)
             self.password = password
         except InvalidPassword as e:
             self._logger.exception(repr(e))