(#4339) Adding empty condition check for the node blog series.

[navsunka]

Closes #4339

[blairlearn]

Note for QA testers: This is related to #4341, and involves deliberately creating an error condition in the database. (#4341 is intended to prevent this condition recurring.)

Testing this will require creating a blog post with no series. If #4341 has already been merged, then this will involve deliberately setting field_blog_series to a non-existent (or deleted) blog series for a selected blog post.

Specifically, use drush sqlc to run the query:

update node_revision__field_blog_series
  set field_blog_series_target_id = SOME NON EXISTENT BLOG SERIES
where entity_id = ID OF THE SELECTED blog post.

[KateMashkinaNIH]

Regression tests pass Test case : ssh into ode and run above query - then regenerated URLs and it succeeded