src/acpigen: support 0-initialized buffer in acpigen_write_byte_buffer

Previously, the 'acpigen_write_byte_buffer' function required both the
byte buffer length and the initialization data byte array 'arr'. The
ACPI spec however allows buffer declarations with only the length, but
without an initialization data byte array. In this case the AML
interpreter will create a buffer of the given length with all bytes
initialized to 0x00. In order to not need another function, allow the
'arr' parameter for the pointer to the initialization data byte array to
be NULL and in that case don't write the optional buffer initialization
byte array.

TEST=Calling 'acpigen_write_byte_buffer' with 'NULL' as first parameter
results in the AML code sequence being written which decompiles to ASL
as expected.

Change-Id: Ie756489e02f994c38d38907a97fb215d30f4a636
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86631
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/acpigen.c

@@ -1775,6 +1775,8 @@ void acpigen_write_to_integer_from_namestring(const char *source, uint8_t dst_op
 	acpigen_emit_byte(dst_op);
 }
 
+/* The initializer byte array 'arr' is optional. When 'arr' is NULL, the AML interpreter will
+   create a 0-initialized byte buffer */
 void acpigen_write_byte_buffer(uint8_t *arr, size_t size)
 {
 	size_t i;
@@ -1783,8 +1785,10 @@ void acpigen_write_byte_buffer(uint8_t *arr, size_t size)
 	acpigen_write_len_f();
 	acpigen_write_integer(size);
 
-	for (i = 0; i < size; i++)
-		acpigen_emit_byte(arr[i]);
+	if (arr != NULL) {
+		for (i = 0; i < size; i++)
+			acpigen_emit_byte(arr[i]);
+	}
 
 	acpigen_pop_len();
 }