snaplist: fix snaplist with filesystem without snaps
snaplist: fix snaplist with filesystem without snaps

When no snaps are seeded in the filesystem, empty string is returned,
which yaml safe load interprets as None.

But snapd seed.yaml is defined as a dictionary with snaps key. Ensure
that when no seed.yaml is present (or readable), that at least an
empty dictionary is returned, such that later calls to `.get("snaps",
[])` succeed.

LP: #1889429

CanonicalLtd/subiquity,CanonicalLtd/subiquity