mb/google/brya/var/{kano,osiris,taeko}: Add null pointer check

Without part no. in CBI, mainboard_get_dram_part_num returns null.
To prevent passing this null pointer to strcmp and avoid unexpected
behavior, proper handling is necessary.

BUG=none
TEST=emerge-brya coreboot

Change-Id: I47e42376c6b1347c56afaec218aed63c5469f0aa
Signed-off-by: Wisley Chen <wisley.chen@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77646
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: David Wu <david_wu@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brya/variants/kano/memory.c

@@ -136,10 +136,11 @@ const struct mb_cfg *variant_memory_params(void)
 	const char *hynix_mem1 = "H54G46CYRBX267";
 	const char *hynix_mem2 = "H54G56CYRBX247";
 
+	if (part_num) {
 		if (!strcmp(part_num, hynix_mem1) || !strcmp(part_num, hynix_mem2)) {
 			printk(BIOS_INFO, "Enable cs_pi_start_high_in_ect for Hynix memory\n");
 			return &hynix_memcfg;
-	} else {
+		}
+	}
 	return &kano_memcfg;
 }
-}

src/mainboard/google/brya/variants/osiris/memory.c

@@ -136,10 +136,11 @@ const struct mb_cfg *variant_memory_params(void)
 	const char *hynix_mem1 = "H54G46CYRBX267";
 	const char *hynix_mem2 = "H54G56CYRBX247";
 
+	if (part_num) {
 		if (!strcmp(part_num, hynix_mem1) || !strcmp(part_num, hynix_mem2)) {
 			printk(BIOS_INFO, "Enable cs_pi_start_high_in_ect for Hynix memory\n");
 			return &hynix_memcfg;
-	} else {
+		}
+	}
 	return &osiris_memcfg;
 }
-}

src/mainboard/google/brya/variants/taeko/memory.c

@@ -135,13 +135,14 @@ static const struct mb_cfg hynix_memconfig = {
 const struct mb_cfg *variant_memory_params(void)
 {
 	const char *dram_part_num = mainboard_get_dram_part_num();
+	if (dram_part_num) {
 		if (strcmp(dram_part_num, "H54G46CYRBX267N") == 0) {
 			printk(BIOS_INFO, "Enable cs_pi_start_high_in_ect for Hynix DRAM part\n");
 			return &hynix_memconfig;
-	} else {
-		return &baseboard_memcfg;
 		}
 	}
+	return &baseboard_memcfg;
+}
 
 int variant_memory_sku(void)
 {