bios: fix bugs in PLDM BIOS

1. Correct json file example for enum type.
2. Enhance conditions for getBIOSAttributeValueTable.

Change-Id: Iba99a2aa0315623baeedec30170739030a6ebb4c
Signed-off-by: Carol Wang <wangkair@cn.ibm.com>
diff --git a/libpldmresponder/bios.cpp b/libpldmresponder/bios.cpp
index 4372e21..e0ded59 100644
--- a/libpldmresponder/bios.cpp
+++ b/libpldmresponder/bios.cpp
@@ -765,7 +765,7 @@
                 }
                 break;
             case PLDM_BIOS_ATTR_VAL_TABLE:
-                if (BIOSAttributeTable.isEmpty())
+                if (BIOSAttributeTable.isEmpty() || BIOSStringTable.isEmpty())
                 {
                     rc = PLDM_BIOS_TABLE_UNAVAILABLE;
                 }