ucm: Do not fail to parse configs on cards with an empty CardComponents lists

ucm: Do not fail to parse configs on cards with an empty CardComponents lists

Since the UCM profiles for all Bay- and Cherry-Trail SST cards have been
moved over to UCM2, parsing them fails with:

ALSA lib ucm_subs.c:220:(uc_mgr_get_substituted_value) variable '${CardComponents}' is not defined in this context!

This completely breaks audio support on all Bay- and Cherry-Trail devices.

This is caused by these non-SOF ASoC using cards having an empty
CardComponents list. Which in itself is fine, but is rejected by
the ucm_subs.c code. This commit changes the ucm_subs code to accept
an empty string as a valid value for CardComponents restoring audio
functionality on these boards.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>