feat: allow PX4_UXRCE_DDS_NS override with empty sting #29777

Job	Run time
build (check_newlines)	1m 34s
build (check_format)	1m 15s
build (tests)	5m 41s
build (tests_coverage)	7m 31s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 16s
build (module_documentation)	1m 10s
build (px4_fmu-v2_default stack_check)	3m 26s
build (shellcheck_all)	1m 47s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 29s
build (px4_sitl_allyes)	4m 31s
build (validate_module_configs)	57s
	35m 37s

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