Initialize typed property "labels" so it can be accessed even if empty (

#69)

* Update IssueField.php

Fix "Typed property JiraCloud\Issue\IssueField::$labels must not be accessed before initialization" when trying to access labels of a retrieved issue.

* Update IssueField.php

Remove whitespace in a blank line

src/Issue/IssueField.php

@@ -102,6 +102,8 @@ class IssueField implements \JsonSerializable
 
     public function __construct($updateIssue = false)
     {
+        $this->labels = [];
+
         if ($updateIssue !== true) {
             $this->project = new \JiraCloud\Project\Project();