XmlImportExport: Fix import error "Column 'profile_id' cannot be null"

In bug_api.php BugData class, the profile_id member variable defaults to
null but the DB bug table does not allow null values, causing the error.

Defaulting the variable to 0 resolves the problem.

Fixes #12112

core/bug_api.php

@@ -100,7 +100,7 @@ class BugData {
 
 	# omitted:
 	# var $bug_text_id
-	protected $profile_id;
+	protected $profile_id = 0;
 
 	# extended info
 	protected $description = '';