fix(xattr): Handle labels without value

I stumbled over this with the very first image I tried:

> sudo systemd-pull oci --system quay.io/toolbx/arch-toolbox:latest

That failed with a label "user.overlay.origin" with a value of NULL and size 0.

This one-line change allows value == NULL if size == 0 when setting xattrs.

(cherry picked from commit 885fe07ee37cff7316680b5088d11081e01813b1)

src/basic/xattr-util.c

@@ -295,7 +295,7 @@ int xsetxattr_full(
         assert(fd >= 0 || fd == AT_FDCWD);
         assert((at_flags & ~(AT_SYMLINK_FOLLOW|AT_EMPTY_PATH)) == 0);
         assert(name);
-        assert(value);
+        assert(value || size == 0);
 
         if (size == SIZE_MAX)
                 size = strlen(value);