ASoC: detect empty DMI strings

Some bootloaders like recent versions of U-Boot may install some DMI
properties with empty values rather than not populate them. This manages
to make its way through the validator and cleanup resulting in a rogue
hyphen being appended to the card longname.

Fixes: 4e01e5d ("ASoC: improve the DMI long card code in asoc-core")
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>
Link: https://patch.msgid.link/20260306174707.283071-2-casey.connolly@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: kcxt

sound/soc/soc-core.c

@@ -1863,12 +1863,15 @@ static void cleanup_dmi_name(char *name)
 
 /*
  * Check if a DMI field is valid, i.e. not containing any string
- * in the black list.
+ * in the black list and not the empty string.
  */
 static int is_dmi_valid(const char *field)
 {
 	int i = 0;
 
+	if (!field[0])
+		return 0;
+
 	while (dmi_blacklist[i]) {
 		if (strstr(field, dmi_blacklist[i]))
 			return 0;