add fix for nullable parent prop in IssueField (#516)

Co-authored-by: tokmanb <[email protected]>

Author: bTokman

src/Issue/IssueField.php

@@ -355,7 +355,7 @@ public function setParentKeyOrId(string $keyOrId): static
         return $this;
     }
 
-    public function setParent(Issue $parent): void
+    public function setParent(?Issue $parent): void
     {
         $this->parent = $parent;
     }