Prevent metadata validation from crashing on missing columns

[sarah-witt]

What does this PR do?

Fixes metadata validation so it does not break when finding missing columns

Motivation

metadata validation currently breaks when displaying errors for integrations extras (error fixed in DataDog/integrations-extras#1384):

/integrations-core/datadog_checks_dev/datadog_checks/dev/tooling/commands/console.py", line 35, in annotate_display_queue
    for func, message in display_queue:
TypeError: cannot unpack non-iterable NoneType object

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 #12680 (728aa2a) into master (70e91b9) will not change coverage.
The diff coverage is n/a.

Flag	Coverage Δ
avi_vantage	91.92% <ø> (ø)
btrfs	82.91% <ø> (ø)
calico	83.33% <ø> (ø)
crio	89.79% <ø> (ø)
ddev	97.99% <ø> (ø)
hazelcast	92.39% <ø> (ø)
ibm_i	81.58% <ø> (ø)

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