FIX update defensive check on tags field method

[HeyImPhil]

Description

If a page type is removed and the page type is orphaned. The report will fail when it is exported to CSV on this data object because it doesn't find the Terms() on SiteTree.

Manual testing steps

1. Delete a page type that is in use (Pages are currently assigned to it)
2. Generate report
3. Export report
4. The report should be generated and downloaded

Issues

• Orphaned page type throws error on Terms method #77

Pull request checklist

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• This change is covered with tests (or tests aren't necessary for this change)
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• CI is green

[GuySartorelli]

LGTM