initialize empty variables array in constructor so that datasources can use the array in explore

public/app/features/templating/template_srv.ts

@@ -17,6 +17,7 @@ export class TemplateSrv {
   constructor() {
     this.builtIns['__interval'] = { text: '1s', value: '1s' };
     this.builtIns['__interval_ms'] = { text: '100', value: '100' };
+    this.variables = [];
   }
 
   init(variables) {