nrf/pwm: Remove superfluous NULL in machine_hard_pwm_instances.

Remove unneeded NULL entry in machine_hard_pwm_instances[] when not
building for NRF52_SERIES.

ports/nrf/modules/machine/pwm.c

@@ -70,8 +70,6 @@ STATIC const nrfx_pwm_t machine_hard_pwm_instances[] = {
 #if NRF52840
     NRFX_PWM_INSTANCE(3),
 #endif
-#else
-    NULL
 #endif
 };