fix mypy issue

Author: ptiurin

src/firebolt/client/auth/base.py

@@ -62,7 +62,7 @@ def account(self) -> Optional[str]:
         return self._account_name
 
     @account.setter
-    def account(self, value: str) -> None:
+    def account(self, value: Optional[str]) -> None:
         self._account_name = value
         # Now we have all the elements to fetch the cached token
         if not self._token: