metrics: Avoid crash when fetching tier metrics

[vadmeste]

Description

Data usage does not always contain tiering info even if the data usage
information is valid. Avoid a crash in that case.

(e.g. the scanner scanned the namespace, the user enables tiering,
prometheus scrapes the server before the scanner gets a chance to
update the data usage with new tiering information)

Motivation and Context

Fix a crash

How to test this PR?

Trivial

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
Deleting image on docker hub
Deleting image locally