osbuild-image-info: don't fail on no systemd default target

Some image types don't have systemd installed and don't have any default
target set, e.g. 'tar' image type. Running osbuild-image-info on such
image would result in traceback (e.g. [1]). Handle this case gracefully.

[1] https://gitlab.com/redhat/services/products/image-builder/ci/osbuild/-/jobs/8911649248#L6480

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

tools/osbuild-image-info

@@ -602,7 +602,10 @@ def read_default_target(tree):
     An example return value:
     "multi-user.target"
     """
-    return subprocess_check_output(["systemctl", f"--root={tree}", "get-default"]).rstrip()
+    try:
+        return subprocess_check_output(["systemctl", f"--root={tree}", "get-default"]).rstrip()
+    except subprocess.CalledProcessError:
+        return ""
 
 
 def read_firewall_default_zone(tree):