input: ft8719: Fix comparing a pointer to a null character constant

../drivers/input/touchscreen/ft8719_spi_c3j/focaltech_test/focaltech_test_ini.c:122:15: warning: comparing a pointer to a null character constant; did you mean to compare to NULL? [-Wpointer-compare]
                if  ((cs == '\0') || (ct == '\0'))
                            ^~~~
                            (void *)0

Signed-off-by: Dede Dindin Qudsy <xtrymind@gmail.com>
Change-Id: Ib27b83946570fe47e74637536fa081c9a9b932bf
Signed-off-by: danascape <danascape@gmail.com>