windows: prevent bytes from being passed to registry APIs There was a TortoiseHg bug report in this area[1], and from inspection, it looks like passing `b""` as `valname` would fail to convert to unicode. The underlying API allows both `""` and `NULL` to return the default value for the key. [1] https://foss.heptapod.net/mercurial/tortoisehg/thg/-/issues/5803