FW LandDetector: disregard horizontal velocity if local_position.v_xy_valid is false #27475

Job	Run time
build (check_format)	1m 12s
build (check_newlines)	1m 37s
build (tests)	5m 46s
build (tests_coverage)	7m 31s
build (px4_fmu-v2_default stack_check)	3m 22s
build (validate_module_configs)	57s
build (shellcheck_all)	1m 35s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 24s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 26s
build (px4_sitl_allyes)	4m 19s
build (module_documentation)	1m 3s
	35m 12s

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