Fix Type Error when a filter has an empty description.

CHANGELOG.md

 # Version 1.78 (not yet released)
 
+* Fix Type Error when a filter has an empty description.
+
 # Version 1.77
 
 * Fix issue with Contact Data Sources in a join.

Civi/DataProcessor/FilterCollection/FilterCollection.php

@@ -96,7 +96,7 @@ class FilterCollection {
     $this->isCollapsed = $isCollapsed;
   }
 
-  public function getDescription(): string {
+  public function getDescription(): ?string {
     return $this->description;
   }