ASoC: soc-utils: Check string pointer validity in snd_soc_dlc_is_dummy()

In the recently added snd_soc_dlc_is_dummy(), the helper uses the .name
and .dai_name fields without checking their validity.

For .name, this field is NULL if the component is matched by .of_node
instead. In fact, only one of these fields may be set. This caused a
NULL pointer dereference on MediaTek MT8195 and MT8188 platforms with
the subsequent conversion to snd_soc_dlc_is_dummy() in their machine
drivers. The codecs are all matches through the device tree, so their
.name fields are empty.

For .dai_name, there are cases where this field is empty, such as for
the following component definitions:

#define COMP_EMPTY() { }
#define COMP_PLATFORM(_name) { .name = _name }
#define COMP_AUX(_name) { .name = _name }
#define COMP_CODEC_CONF(_name) { .name = _name }

Or the single link CPU DAI case in the simple audio card family, as
covered by simple_util_canonicalize_cpu(), in which the .dai_name
field is explicitly cleared.

To fix this, check the validity of the fields before using them in
string comparison.

Fixes: 3e021f3b8115 ("ASoC: soc-utils: add snd_soc_dlc_is_dummy()")
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/20250516050549.407133-1-wenst@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>