core/cgroup: fix compile error

core/cgroup: fix compile error

With gcc-13,
```
CFLAGS="-O3 -fno-semantic-interposition" meson setup build
```
triggers the following error:
```
../src/core/cgroup.c: In function ‘cgroup_context_dump’:
../src/core/cgroup.c:633:44: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  633 |                         "%sDeviceAllow: %s %s\n",
      |                                            ^~
cc1: some warnings being treated as errors
```

Fixes #30223.