Islandora/islandora#2338: Handle unset structured_text_term_uri option

[xurizaemon]

GitHub Issue: Islandora/documentation#2338

What does this Pull Request do?

Prevents Islandora emitting a warning when the structured_text_term_uri option is not configured in iiif_manifest view REST export.

What's new?

After viewing content via Mirador, mapped to node/%/book-manifest, when structured_text_term_uri and search_endpoint fields are null (default state), a warning is displayed or logged:

Warning: Undefined array key "structured_text_term_uri" in Drupal\islandora_iiif\Plugin\views\style\IIIFManifest->getStructuredTextTerm() (line 714 of /app/web/modules/contrib/islandora/modules/islandora_iiif/src/Plugin/views/style/IIIFManifest.php)

How should this be tested?

Steps to reproduce behaviour:

1. At /admin/config/development/logging set " Error messages to display " to "All messages, with backtrace information"
2. At /admin/structure/views/view/iiif_manifest/edit/rest_export_3, do not have structured_text_term_uri field set (default state).
3. View an image in Mirador viewer
4. Reload the page, or click to another page
5. Error should be displayed

Steps to test:

1. Apply patch
2. Do steps to reproduce, do not see error at 5

Documentation Status

No docs required

Additional Notes:

n/a

Interested parties

@adam-vessey, @kayakr

[xurizaemon]

I wonder about the need for $this->structuredTextTermMemoized at all, when we could instead check if $this->structuredTextTerm is NULL, but that's another issue.

However, this change also modifies the behaviour slightly:

• Previously, if the lookup failed then $this->structuredTextTermMemoized would be TRUE and $this->structuredTextTerm would be NULL.
• With this change, if the lookup failed then $this->structuredTextTermMemoized will be FALSE and $this->structuredTextTerm will remain NULL.

That looks more correct to me, but I'm new here, so please consider that behaviour also!

[xurizaemon]

Closing, see #1047 (comment)