statistics: Fix warning on unset fieldPresentation.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2171 44740490-163a-0410-bde0-09ae8108e29a

modules/statistics/lib/StatDataset.php

@@ -272,7 +272,7 @@ class sspmod_statistics_StatDataset {
 			return $presentationHandler->getPresentation();
 		}
 
-		return NULL;
+		return array();
 	}
 	
 	public function getDelimiterPresentationPie() {