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
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