DAOS-16026 pydaos: fix offset issue in oit_mark

[johannlombardi]

Fixing offset when appending a \0 between keys oit_mark().

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[github-actions]

Ticket title is 'pydaos string termination issue'
Status is 'Open'
https://daosio.atlassian.net/browse/DAOS-16026

[daosbuild1]

Test stage Functional on EL 8.8 completed with status FAILURE. https://build.hpdd.intel.com//job/daos-stack/job/daos/view/change-requests/job/PR-14570/1/execution/node/1208/log