Deserialization errors in Store::restore_session() after matrix-sdk update

After updating matrix-sdk `Client::restore_login()` resp. `Store::restore_session()` fails:
```
thread 'main' panicked at 'Error restoring session:: StateStore(Json(Error("expected value", line: 1, column: 1)))', src/main.rs:212:151
```

As a workaround remove the state store:
```
rm -r ~/.local/share/matrixmail/matrix-sdk-state
```