bindings/c: Don't forward empty encryption key string

op-sqlite, for example, always passes the remote encryption key:

```
DB opsqlite_libsql_open_sync(std::string const &name,
                             std::string const &base_path,
                             std::string const &url,
                             std::string const &auth_token, int sync_interval,
                             bool offline, std::string const &encryption_key,
                             std::string const &remote_encryption_key) {
```

Therefore, ensure that remote encryption key is not null and non-empty
before passing it as part of HTTP requests.