cli/show: If a leaf part has children, show them instead of omitting

Until we did PKCS#7 unwrapping, no leaf MIME part could have a child.

Now, we treat the unwrapped MIME part as the child of the PKCS#7
SignedData object.  So in that case, we want to show it instead of
deliberately omitting the content.

This fixes the test of the protected subject in
id:smime-onepart-signed@protected-headers.example.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>