ekf2: fix optical_flow_vel publication #20995

Job	Run time
build (check_format)	1m 21s
build (tests)	10m 19s
build (tests_coverage)	10m 37s
build (px4_fmu-v2_default stack_check)	4m 20s
build (validate_module_configs)	1m 3s
build (shellcheck_all)	2m 47s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	5m 57s
build (NO_NINJA_BUILD=1 px4_sitl_default)	5m 39s
build (airframe_metadata)	1m 17s
build (module_documentation)	1m 22s
build (parameters_metadata)	2m 1s
	46m 43s

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