Fix init failure when auth_ticket is not provided

[coignetp]

What does this PR do?

auth_ticket is not required for all environment, and assigning an environment variable with None raises an exception.

Motivation

Support case

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #9390 (4181f47) into master (02c6201) will not change coverage.
The diff coverage is 100.00%.

Flag	Coverage Δ
mapr	84.97% <100.00%> (ø)

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Impacted Files	Coverage Δ
mapr/datadog_checks/mapr/mapr.py	69.07% <100.00%> (ø)