Update variationKey type to be string

Author: yavorona

packages/optimizely-sdk/lib/core/event_builder/event_helpers.d.ts

@@ -18,7 +18,7 @@ import { EventTags, UserAttributes } from '../../shared_types';
 
 interface ImpressionConfig {
   experimentKey: string;
-  variationKey: string | null;
+  variationKey: string;
   flagKey: string;
   ruleType: string;
   userId: string;

packages/optimizely-sdk/lib/core/project_config/index.js

@@ -297,7 +297,7 @@ export var getVariationKeyFromId = function(projectConfig, variationId) {
  */
 export var getVariationIdFromExperimentAndVariationKey = function(projectConfig, experimentKey, variationKey) {
   var experiment = projectConfig.experimentKeyMap[experimentKey];
-  if (variationKey && experiment.variationKeyMap.hasOwnProperty(variationKey)) {
+  if (variationKey !== '' && experiment.variationKeyMap.hasOwnProperty(variationKey)) {
     return experiment.variationKeyMap[variationKey].id;
   }
   return null;

packages/optimizely-sdk/lib/optimizely/index.ts

@@ -348,7 +348,7 @@ export default class Optimizely {
     const impressionEvent = getImpressionEvent(impressionEventOptions);
     const experiment = configObj.experimentKeyMap[experimentKey];
     let variation;
-    if (experiment && experiment.variationKeyMap && variationKey) {
+    if (experiment && experiment.variationKeyMap && variationKey !== '') {
       variation = experiment.variationKeyMap[variationKey];
     }
     this.notificationCenter.sendNotifications(NOTIFICATION_TYPES.ACTIVATE, {