Prevent PushTicket/PushReceipt jsonSerialize() stripping all falsy values (#86)

`PushTicket::jsonSerialize()` and `PushReceipt::jsonSerialize()` now
explicitly filter out only `null` values, matching the predicate already
used by `PushMessage::jsonSerialize()`.

Fixes #73

Author: dru1x

src/PushReceipt/PushReceipt.php

@@ -38,6 +38,7 @@ public function jsonSerialize(): array
     {
         return array_filter(
             get_object_vars($this),
+            fn(mixed $value): bool => !is_null($value),
         );
     }
 }

src/PushTicket/PushTicket.php

@@ -39,6 +39,7 @@ public function jsonSerialize(): array
     {
         return array_filter(
             get_object_vars($this),
+            fn(mixed $value): bool => !is_null($value),
         );
     }