Set default values for changelog + fixVersions (#543)

* set default value for changelog

* set default value for fixVersions

Author: hgriga

src/Issue/IssueField.php

@@ -47,7 +47,7 @@ class IssueField implements \JsonSerializable
 
     public ?object $resolution = null;
 
-    public array $fixVersions;
+    public array $fixVersions = [];
 
     public ?Reporter $creator = null;