Initialize sample buffer pointer with nullptr #38386

Job	Run time
build (stm32f7, chibios, 10)	4m 4s
build (stm32h7, chibios, 10)	3m 23s
build (fmuv2-plane, chibios, 10)	2m 0s
build (periph-build, chibios, 10)	3m 44s
build (iofirmware, chibios, 10)	1m 32s
build (CubeOrange-bootloader, chibios, 10)	50s
build (fmuv3-bootloader, chibios, 10)	48s
build (revo-bootloader, chibios, 10)	58s
build (stm32h7-debug, chibios, 10)	2m 7s
build (fmuv3, chibios, 10)	5m 1s
build (revo-mini, chibios, 10)	4m 20s
build (MatekF405-Wing, chibios, 10)	4m 10s
build (CubeOrange-ODID, chibios, 10)	2m 32s
build (CubeRedPrimary-bootloader, chibios, 10)	49s
build (configure-all, chibios, 10)	9m 34s
build (build-options-defaults-test, chibios, 10)	7m 32s
build (signing, chibios, 10)	2m 42s
build (CubeOrange-PPP, chibios, 10)	2m 2s
build (SOHW, chibios, 10)	3m 21s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 23s
	1h 2m 52s

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