Fix build error caused by blog tags not being a list

[pedro-psb]

This is hitting the CI if the frontmatter (yaml) doesnt contain a list in the 'tags` field.

ERROR   -  Error reading page 'blog/posts/2021/2021-06-28-tasking-event-blog.md':
ERROR   -  Error reading tags of page 'blog/posts/2021/2021-06-28-tasking-event-blog.md' in '../../../../../../tmp/pulp-docs-tmp/repo_docs':
Expected iterable tags, but received: event

Aborted with a BuildError!

Couldnt reproduce locally.
Maybe some dependency changed how it handles this.