find_legacy_keymap: fix empty variant matching

find_legacy_keymap: fix empty variant matching

We should give a match bonus if the X context variant is empty
and the xvariant column in kbd-model-map is "-" (which means
none). Currently, we don't, which means that if you call this
on a context with layouts bg,us and no variant, you get the
console layout bg_pho-utf8 instead of bg_bds-utf8 (because both
score the same, and the bg_pho-utf8 row comes first). You should
get bg_bds-utf8 in this case.

Signed-off-by: Adam Williamson <awilliam@redhat.com>