OCI: Handle NULL when marshalling manifest string maps
OCI: Handle NULL when marshalling manifest string maps

This was happening for the org.opencontainers.image.ref.name annotation
when we're in labels mode, but might as well be handled in a generic
way.

Closes: #2978
Approved by: alexlarsson

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak