drivers/pc80/vga: Add NULL check for `vga_line_write()`

This patch ensures vga_line_write() returns if the argument 1
(aka output string) is NULL.

TEST=Able to build and boot Google/Taeko.

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: I716ce82c0afe21f7fe2f6d7bdc5229f8087242fa
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71264
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tarun Tuli <taruntuli@google.com>

src/drivers/pc80/vga/vga.c

@@ -262,6 +262,9 @@ vga_frame_set(unsigned int line, unsigned int character)
 void
 vga_line_write(unsigned int line, const char *string)
 {
+	if (!string)
+		return;
+
 	unsigned short *p = (unsigned short *)VGA_FB + (VGA_COLUMNS * line);
 	size_t i, len = strlen(string);