Issue #3484946 by scott_earnest: Declaration of...

Issue #3484946 by scott_earnest: Declaration of Drupal\facets\Entity\Facet::__sleep() must be compatible

Closes #3484946

src/Entity/Facet.php

@@ -1217,7 +1217,7 @@ class Facet extends ConfigEntityBase implements FacetInterface {
   /**
    * Remove the facet lazy built data when the facet is serialized.
    */
-  public function __sleep() {
+  public function __sleep(): array {
     unset($this->facet_source_instance);
     unset($this->processors);