[mongo] fix db name parsing when namespace has no collection

[lu-zhengda]

What does this PR do?

This PR fixes a bug in parsing db name from namespace when namespace has no collection name, e.g. admin.$cmd vs. admin. This usually happens with database level administration command on earlier MongoDB version (version 5 or earlier).

Motivation

https://datadoghq.atlassian.net/browse/DBMON-4723

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• [ ] If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.30%. Comparing base (ce674ce) to head (0423e2b). Report is 1 commits behind head on master.

Additional details and impacted files

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