tests: umockdev-record: t_system_single: handle missing SELinux context on /dev/null #1136

Job	Run time
OS (apt-docker.io/amd64/ubuntu:devel)	2m 10s
OS (apt-docker.io/amd64/ubuntu:latest)	2m 9s
OS (apt-docker.io/amd64/ubuntu:rolling)	2m 2s
OS (apt-docker.io/i386/debian:testing)	1m 54s
OS (apt-docker.io/amd64/debian:testing)	2m 10s
OS (dnf-registry.fedoraproject.org/fedora:latest)	1m 54s
OS (dnf-registry.fedoraproject.org/fedora:rawhide)	2m 8s
OS (alpine-docker.io/alpine)	27s
OS (alpine-docker.io/i386/alpine)	38s
OS ([email protected]/alpine)	27s
OS (nix-docker.io/nixos/nix)	1m 15s
OS (gentoo-)	3m 52s
OS (alpine@gudev-alpine, EXTRA_PACKAGES=libgudev-dev)	33s
	21m 39s

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