Allow resetting multipleOf back to null

Author: bertramakers

src/Schema/Type/Number.php

@@ -90,7 +90,7 @@ public function maximum(?float $maximum, bool $exclusive = false): static
 
     public function multipleOf(?float $number): static
     {
-        if ($number <= 0) {
+        if ($number !== null && $number <= 0) {
             throw new InvalidArgumentException('multipleOf must be a positive number');
         }