Allow adding custom field with NULL to clear it (#48)

* Allow adding custom field with NULL to clear it.

* Style fix for type info.

Author: DennisBetawerk

src/Issue/IssueField.php

@@ -155,7 +155,7 @@ public function getCustomFields(): ?array
         return $this->customFields;
     }
 
-    public function addCustomField(string $key, string|int|float|array|Document $value): static
+    public function addCustomField(string $key, string|int|float|array|Document|null $value): static
     {
         $this->customFields[$key] = $value;