AceDatabase: AceConverter accessors return undef rather than empty list.

Fixes RT367947.
New release tag humpub-release-78-04

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New release tag humpub-release-78-05

Generated by /nfs/users/nfs_m/mca/gitwk-anacode/team_tools.stable/otterlace/release/scripts/otterlace_release_tag (otter/75-101-g84dd131 from intcvs1:/repos/git/anacode/team_tools.git)