boot: android: Check kcmdline's for NULL in android_image_get_kernel()

boot: android: Check kcmdline's for NULL in android_image_get_kernel()

kcmdline and kcmdline_extra strings can be NULL. In that case, we still
read the content from 0x00000 and pass that to the kernel, which is
completely wrong.

Fix android_image_get_kernel() to check for NULL before checking if
they are empty strings.

Fixes: 53a0ddb6d3be ("boot: android: fix extra command line support")
Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
Reviewed-by: Nicolas Belin <nbelin@baylibre.com>
Reviewed-by: Julien Masson <jmasson@baylibre.com>
Tested-by: Sam Day <me@samcday.com>
Link: https://lore.kernel.org/r/20250113-kcmdline-extra-fix-v1-1-03cc9c039159@baylibre.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>