Return early from setupHubPromoABTest if A/B tests have not yet been …

…fetched to avoid misinterpreting absense of 'hub_plain' or 'hub_midnight'

src/background.js

@@ -1118,7 +1118,12 @@ function getAntitrackingTestConfig() {
  * @return {Object} 	Hub promotion configuration parameters
  */
 function setupHubPromoABTest() {
-	if (conf.hub_promo_variant !== 'not_yet_set') return;
+	if (
+		!abtest.hasBeenFetched
+		|| conf.hub_promo_variant !== 'not_yet_set'
+	) {
+		return;
+	}
 
 	if (abtest.hasTest('hub_plain')) {
 		conf.hub_promo_variant = 'plain';

src/classes/ABTest.js

@@ -27,6 +27,7 @@ const { BROWSER_INFO, CMP_BASE_URL, EXTENSION_VERSION } = globals;
 class ABTest {
 	constructor() {
 		this.tests = {};
+		this.hasBeenFetched = false;
 	}
 
 	/**
@@ -63,6 +64,7 @@ class ABTest {
 					(tests, test) => Object.assign(tests, { [test.name]: test.data }),
 					{}
 				);
+				this.hasBeenFetched = true;
 			} else {
 				log('A/B Tests: no tests found.');
 			}