CffMapping: Add checks for value types

For example, the crash in https://sentry.softwareheritage.org/share/issue/a67ff03ed63448d0bf386f832564f380/

is caused by @id being None because of https://archive.softwareheritage.org/swh:1:cnt:b7d98a32465e15415619d9b40ed6c58e218d78ad;origin=https://github.com/int-brain-lab/ibllib;visit=swh:1:snp:9027c747eafbdeb342f2c0a32b9374a850014567;anchor=swh:1:rev:e4a4f8895efb2d3a0ae5097ee619b1629ee6c537;path=/CITATION.cff;lines=6

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Migrated from T4275 (view on Phabricator)