navigator: Initialize the message variable #25039

Job	Run time
build (check_format)	1m 3s
build (check_newlines)	1m 27s
build (tests)	5m 37s
build (tests_coverage)	7m 23s
build (px4_fmu-v2_default stack_check)	3m 14s
build (validate_module_configs)	54s
build (shellcheck_all)	1m 39s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 17s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 19s
build (px4_sitl_allyes)	4m 22s
build (airframe_metadata)	1m 4s
build (module_documentation)	1m 6s
build (parameters_metadata)	1m 14s
	36m 39s

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