Makes Typesense config option default_sorting_field optional

• Typesense v0.20.0 made this optional. This change follows suit for Drupal.
• The change doesn't prevent default_sorting_field from being written to the Typesense schema, but Typesense correctly interprets the empty string it's set to.
• Re: 3222119-make-default-sorting-field-optional

Closes #3222119