Fixes #18816 - no longer rely on nil/false for default capsule

[jlsherrill]

and instead require that a capsule object be present,
and default to the default capsule

[Klaas-]

Hi,
I've tested the patch and after applying it (plus #6622 and #6649 as prereqs on 3.3.0) manifest import and sync work normally again. I had to rerun "foreman-rake katello:upgrades:3.0:add_export_distributor" to get my non-redhat repositories back into a working state

Thanks.

[ehelms]

Thanks @jlsherrill !