AP_GPS: Protect against nullptr port #48336

Job	Run time
build (fmuv2-plane, chibios, 10)	2m 2s
build (periph-build, chibios, 10)	3m 45s
build (stm32h7, chibios, 10)	3m 16s
build (iofirmware, chibios, 10)	1m 30s
build (stm32f7, chibios, 10)	3m 39s
build (fmuv3, chibios, 10)	5m 29s
build (MatekF405-Wing, chibios, 10)	4m 32s
build (build-options-defaults-test, chibios, 10)	6m 40s
build (revo-bootloader, chibios, 10)	50s
build (CubeOrange-ODID, chibios, 10)	2m 38s
build (CubeRedPrimary-bootloader, chibios, 10)	51s
build (fmuv3-bootloader, chibios, 10)	1m 7s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 13s
build (new-check, chibios, 10)	4m 8s
build (revo-mini, chibios, 10)	4m 8s
build (configure-all, chibios, 10)	11m 9s
build (CubeRed-EKF2, chibios, 10)	2m 17s
build (SOHW, chibios, 10)	3m 11s
build (stm32h7-debug, chibios, 10)	2m 11s
build (CubeOrange-bootloader, chibios, 10)	54s
build (signing, chibios, 10)	2m 57s
build (CubeOrange-PPP, chibios, 10)	1m 59s
	1h 10m 26s

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