staging: atomisp: Removes pointer comparison to 0 in ia_css_queue_get_size

Repaces pointer comparison to 0 with NULL in ia_css_queue_get_size
to improve code readability. Identified with coccinelle script
'badzero.cocci'.

Signed-off-by: Tahia Khan <tahia.khan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>