Do not render the pdf_url metatag if there is no value

[joecorall]

What does this Pull Request do?

With the space added to the [islandoratokens:pdf_url] token, if that token is used in metatags, the <meta> element always renders even if there is no PDF associated with the given node.

Also updates the function that token uses to always return a string.

What's new?

Remove the space from the token.

How should this be tested?

• Enable the metatags module
• Add [islandoratokens:pdf_url] to one of the metatag elements (for testing purposes, it can be any element)
• Visit a node/XYZ page on your site that does not have a PDF
• View the HTML source and see <meta name="citation_pdf_url" content=" " />
• Apply this PR
• See that that element no longer shows

Documentation Status

• Does this change existing behaviour that's currently documented? no
• Does this change require new pages or sections of documentation? no
• Who does this need to be documented for? no

Interested parties

@Islandora/committers
@wgilling

[wgilling]

Enable the metatags module
I tried to test this and I got stumped by the first step. Should I run composer require drupal/metatags followed by running drush pm:enable metatags?

[joecorall]

@wgilling I tried to test this and I got stumped by the first step. Should I run composer require drupal/metatags followed by running drush pm:enable metatags?

That sounds right. However you normally install/enable modules will do.

[wgilling]

Ok - sorry for the false alarm there. With the original islandora 2.x code, I cloned https://git.drupalcode.org/project/metatag off in a separate folder and then copied it under the modules/contrib and then enabled it using the GUI. I then proceeded to add a meta tag for original_source: | [islandoratokens:pdf_url] and navigated to a node which did not have any document pdf and I saw the meta tag with the empty content part of it:
<meta name="original-source" content="" />

I then swapped out the islandora code with Joe's branch and cleared the cache and viewed the same object as before and view-source revealed that this metatag was no longer rendered, so this PASSES QC and I'd approve a merge of this.

[wgilling]

This change passed the steps to test and I approve to merge.