password unification refactor: move methods from wallet to daemon

Note in particular that check_password_for_directory was not safe to use while the daemon had wallets loaded,
as the same file would have two corresponding Wallet() instances in memory. This was specifically handled in
the kivy GUI, on the caller side, by stopping-before and reloading-after the wallets; but it was dirty to
have the caller handle this.

electrum/storage.py

@@ -146,6 +146,8 @@ class WalletStorage(Logger):
 
     @staticmethod
     def get_eckey_from_password(password):
+        if password is None:
+            password = ""
         secret = hashlib.pbkdf2_hmac('sha512', password.encode('utf-8'), b'', iterations=1024)
         ec_key = ecc.ECPrivkey.from_arbitrary_size_secret(secret)
         return ec_key
@@ -160,6 +162,7 @@ class WalletStorage(Logger):
             raise WalletFileException('no encryption magic for version: %s' % v)
 
     def decrypt(self, password) -> None:
+        """Raises an InvalidPassword exception on invalid password"""
         if self.is_past_initial_decryption():
             return
         ec_key = self.get_eckey_from_password(password)