Fix error with NULL sample handling in `audiofilters.Filter`. #15627

Job	Run time
scheduler	1m 54s
mpy-cross-mac	54s
windows	0s
docs	0s
tests / run (all)	49s
tests / run (mpy)	35s
tests / run (native)	49s
mpy-cross / build (static)	16s
tests / run (native_mpy)	41s
mpy-cross / build (static-aarch64)	32s
mpy-cross / build (static-mingw)	1m 12s
mpy-cross / build (static-raspbian)	41s
ports (raspberrypi) / board (adafruit_feather_rp2350)	2m 23s
ports (raspberrypi) / board (adafruit_metro_rp2350)	2m 9s
ports (raspberrypi) / board (challenger_rp2350_bconnect)	2m 34s
ports (raspberrypi) / board (challenger_rp2350_wifi6_ble5)	2m 13s
ports (raspberrypi) / board (cytron_iriv_io_controller)	2m 20s
ports (raspberrypi) / board (cytron_motion_2350_pro)	2m 22s
ports (raspberrypi) / board (datanoise_picoadk_v2)	2m 27s
ports (raspberrypi) / board (pimoroni_pga2350)	2m 25s
ports (raspberrypi) / board (pimoroni_pico_plus2)	2m 24s
ports (raspberrypi) / board (pimoroni_plasma2350)	2m 13s
ports (raspberrypi) / board (pimoroni_tiny2350)	2m 19s
ports (raspberrypi) / board (raspberry_pi_pico2)	2m 28s
ports (raspberrypi) / board (seeeduino_xiao_rp2350)	2m 23s
ports (raspberrypi) / board (solderparty_rp2350_stamp)	2m 29s
ports (raspberrypi) / board (solderparty_rp2350_stamp_xl)	2m 12s
ports (raspberrypi) / board (sparkfun_pro_micro_rp2350)	2m 12s
	45m 56s

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