Update Rule.php

Sometimes legacy data could trigger an exception because of null-values.

src/PricingManager/Rule.php

@@ -120,7 +120,7 @@ public function setValue(string $key, mixed $value, bool $ignoreEmptyValues = fa
                         return $this;
                     }
             }
-            $this->$method($value);
+            $this->$method($value ?? '');
         }
 
         return $this;