[SNMP] Fix metric length checking

[nmuesch]

What does this PR do?

Check if the metrics field exists, not its length.

Motivation

If there is nothing there, its actually None so checking its length just throws an exception.

Testing Guidelines

An overview on testing
is available in our contribution guidelines.

Versioning

Resolves issue in unreleased check.

• Bumped the check version in manifest.json
• Bumped the check version in datadog_checks/{integration}/__init__.py
• Updated CHANGELOG.md. Please use Unreleased as the date in the title
  for the new section.
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Anything else we should know when reviewing?