xtrabackup: handle missing facts gracefully

[fraenki]

Summary

When setting up a new server (or when reinstalling an existing server) with xtrabackup enabled, the Puppet Run may fail:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, 'versioncmp' parameter 'a' expects a String value, got Undef (file: /etc/puppetlabs/code/environments/development/modules/mysql/manifests/backup/xtrabackup.pp, line: 53, column: 8) on node 

This patch adds a check to verify that the facts are available, before accessing them.

Additional Context

none

Related Issues (if any)

none

Checklist

• 🟢 Spec tests.
• 🟢 Acceptance tests.
• Manually verified. (For example puppet apply)

[fraenki]

rebased