[kcms/keyboard] Avoid serializing list options as \\0

VariantList, DisplayNames and Options are comma-delimited string lists in a
setxkbmap/libxkbcommon compatible format, but serializing a QStringList with
a single empty string ends up as "\\0" in the config file, which is not
compatible. This can't happen for LayoutList, because layout strings are never
empty.

Add a kconf_update script to convert \\0 into empty lists for the affected
entries.

This restores the old format for compatibility with e.g. KWin.

BUG: 433265