Allow null values (#527)

e.g. to remove a previous selection from a checkbox field

src/Issue/IssueField.php

@@ -150,7 +150,7 @@ public function getCustomFields(): ?array
  return $this->customFields;
  }
 
- public function addCustomField(string $key, string|int|float|array $value): static
+ public function addCustomField(string $key, null|string|int|float|array $value): static
  {
  $this->customFields[$key] = $value;