Pivot creates empty json when using one value in a multi value field

Because pivot is used, the pivot function returns empty array, as the count is zero. This happens because the explode function does not return an array. After removing lines in code that return a string instead of array if there is only one element in array, the pivot on multi value field which only has one value added no longer returns an empty array in the ld+Json, but the actual data. Because it is still marked as multi value field because of pivot it is still "nested" in array, so not a "format" as without the pivot. Not sure if this breaks anything elsewhere. It needs manual testing and maybe also additional tests need to be written. I'm also attaching a patch besides the commit.

Besides the test needing to be updated, if this is the right approach, I think this now also "breaks" the FAQ rendering if there is an empty value for Q and A. It renders the "empty" main entity, something like:

"@type": "FAQPage",
            "mainEntity": [
                {
                    "@type": "Question",
                    "answerCount": "1",
                    "acceptedAnswer": {
                        "@type": "Answer"
                    }
                }
            ]

My updated "quick" fix, that should only work for FAQ is to add additional check in the arrayTrim function in SchemaMetatagManager.php:

   // Added before the foreach
   $countBefore = count($array);

    // Specific for FAQ, if the answer array changes (text field is removed)
    // it means it's empty, so remove the array.
    if ($countBefore > count($array) && array_key_last($array) === 'pivot') {
      return [];
    }

    // Specific for FAQ, if the last key is answerCount it means the question
    // is empty so nothing to return.
    if (count($array) == 3 && array_key_last($array) === 'answerCount') {
      return [];
    }
   
    }

Not sure what this will break, so not committing the code just yet, but leaving it here if anybody also gets across this issue and has any better solution.

The #5/MR!14 patch causes the metatag output to also create arrays where it shouldn't. E.g., when I apply the patch an ImageObject without pivot enabled gets turned in to:

"image": {
  "@type": ["ImageObject"],
  "representativeOfPage": ["True"],
  "url": ["{URL}"],
  "width": ["1424"],
  "height": ["801"]
}

So this needs work for more than just tests.

I have opened MR!15 with a different approach that tries to address this specific issue (a single entity in a pivot-configured field) by checking for the condition in \Drupal\schema_metatag\SchemaMetatagManager::pivot (where we already know a pivot is happening). If pivotable items are not found at that point, the change in MR!15 just returns the original item value.

Rerolling for 3.0.x.

Committed. Thank you.