pkglist: Canonicalize "no packages" to "no database found"

[cgwalters]

It can happen that we successfully query zero packages. For example,
when rpm-ostree on RHEL8 (assuming bdb database) is trying to parse an
ostree commit generated from fedora (sqlite rpmdb), librpm will just give us
nothing. Eventually perhaps we may need to fall back to actually running
the target commit as a container just to get this data for cases like that.

Specifically, I hit this doing
rpm-ostree rebase --experimental ostree-unverified-image:registry:quay.io/cgwalters/fcos-derivation-example
on RHCOS8.

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[cgwalters]

Only compile tested locally so
/test all
Now, clearly a few things needed here:

• Need a test case
• The higher level code around print_treepkg_diff_from_sysroot_path() in particular needs to more gracefully say e.g. Unable to compute package diff; no rpmdb found in commit %s or so.

[cgwalters]

/test all

[openshift-ci]

@cgwalters: PR needs rebase.

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[openshift-ci]

@cgwalters: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/build-clang	9369c6a	link	true	/test build-clang
ci/prow/clang-analyzer	9369c6a	link	true	/test clang-analyzer
ci/prow/images	9369c6a	link	true	/test images
ci/prow/fcos-e2e	9369c6a	link	true	/test fcos-e2e
ci/prow/kola-upgrade	9369c6a	link	true	/test kola-upgrade

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.