TypeError Highlight::prepareTextForExcerpt()

Problem/Motivation

If a field to index of a search result is empty, an error occurs because Highlight::prepareTextForExcerpt() expects a string as parameter:

TypeError: Drupal\search_api\Plugin\search_api\processor\Highlight::prepareTextForExcerpt(): Argument #1 ($text) must be of type string, null given, called in /var/www/html/web/modules/contrib/search_api/src/Plugin/search_api/processor/Highlight.php on line 898 in Drupal\search_api\Plugin\search_api\processor\Highlight->prepareTextForExcerpt() (line 678 of modules/contrib/search_api/src/Plugin/search_api/processor/Highlight.php).

Drupal\search_api\Plugin\search_api\processor\Highlight->createFallbackExcerpt(Array, 256) (Line: 550)
Drupal\search_api\Plugin\search_api\processor\Highlight->createExcerptForFields(Array, Array) (Line: 321)
Drupal\search_api\Plugin\search_api\processor\Highlight->addExcerpts(Array, Array, Array) (Line: 273)
Drupal\search_api\Plugin\search_api\processor\Highlight->postprocessSearchResults(Object) (Line: 771)
Drupal\search_api\Entity\Index->postprocessSearchResults(Object) (Line: 620)
Drupal\search_api\Query\Query->postExecute() (Line: 552)
Drupal\search_api\Query\Query->execute() (Line: 622)
Drupal\search_api\Plugin\views\query\SearchApiQuery->execute(Object) (Line: 1513)

Steps to reproduce

• Create a taxonomy term with empty description
• Set the corresponding vocabulary as datasource
• Add the description to the fields to index
• Search for the taxonomy term title

Proposed resolution

Check if the field value is a string before calling Highlight::prepareTextForExcerpt().