Fix undefined variable (#6698)

Author: benmccann

Diff for: src/core/core.scale.js

@@ -356,7 +356,7 @@ var Scale = Element.extend({
 	*/
 	_getLabels: function() {
 		var data = this.chart.data;
-		return this.options.labels || (this.isHorizontal() ? data.xLabels : data.yLabels) || data.labels;
+		return this.options.labels || (this.isHorizontal() ? data.xLabels : data.yLabels) || data.labels || [];
 	},
 
 	// These methods are ordered by lifecyle. Utilities then follow.