Avoid js TypeError when no BYG pages are configured

The getMatchingAudiences function expects urls to be passed as an array,
but if there are no URLs configured for the site, that value is null.
This sets a sensible default to avoid console warnings bootstrapping the
BYG frontend.

src/frontend.js

@@ -27,9 +27,11 @@ import { injectPage } from './services/history';
 const defaultTrigger = ( BYG ) => {
 	const { callback, urls } = BYG;
 
-	BYG.url = getMatchingAudience( urls );
+	if ( callback && urls ) {
+		BYG.url = getMatchingAudience( urls );
 
-	callback( BYG );
+		callback( BYG );
+	}
 };
 
 /**
@@ -50,6 +52,7 @@ window.addEventListener( 'load', () => {
 	const BYG = Object.assign( {
 		referrers: [],
 		utmSources: [],
+		urls: [],
 	}, window.BYG || {} );
 
 	if ( typeof BYG.trigger === 'function' ) {