manager: avoid `KeyError` in `run_getmetadata`
Part of removing base data on ERP5 Documents, I wanted to systematically run run_getmetadata to get metadata of a document, previously extracted from the call to run_convert to base data. I got an error for documents where title was not set, which apparently comes from a Cloudooo backward-compatibility feature.
I also noticed, but not fixed in this MR, a discrepancy between run_convert and run_getmetadata: one has backward compatibility for MIMEType, and the other for Title. I think we should unify, what do you think of moving these to getFileMetadataItemList?