ISBN in certain cases null when using Pivot

We are also noticing the same issue when pivot is enabled on a tag with multiple tokens and if any, but the last, token returns no value, this error is thrown:
Notice: Undefined offset: 0 in Drupal\schema_metatag\SchemaMetatagManager::pivot() (line 140 of /var/www/web/modules/contrib/schema_metatag/src/SchemaMetatagManager.php)

In SchemaNameBase::processItem(), the value is exploded into an array, and any tokens with no value will be included in that position of the array as an empty element:

if ($explode) {
  $value = $this->schemaMetatagManager()->explode($value);
  // Clean out any empty values that might have been added by explode().
  if (is_array($value)) {
    $value = (array_filter($value);
  }
}

$value = array_filter($value); does remove all the empty elements from the array, but it does not reindex the array to fill in the indices of the removed empty elements. This would need to be updated to $value = array_values(array_filter($value)); for the reindexing.

For example, if a tag was mapped with [node:field_foo],[node:field_bar] where [node:field_foo] returns no value, the resulting value array:

$value = [
  0 => "",
  1 => "Some value",
];

would become:

$value = [
  1 => "Some value",
];

instead of the expected:

$value = [
  0 => "Some value",
];

The attached patch should handle this appropriately.

This looks reasonable.

Committed. Thank you.