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

mercurial/windows.py

@@ -680,7 +680,9 b' def lookupreg(key, valname=None, scope=N'
             # pytype: disable=module-attr
             with winreg.OpenKey(s, encoding.strfromlocal(key)) as hkey:
                 # pytype: enable=module-attr
-                name = valname and encoding.strfromlocal(valname) or valname
+                name = None
+                if valname is not None:
+                    name = encoding.strfromlocal(valname)
                 # pytype: disable=module-attr
                 val = winreg.QueryValueEx(hkey, name)[0]
                 # pytype: enable=module-attr