Export NullAnalytics from main package

  Fixes #1336

  NullAnalytics was accidentally removed from the public exports in
  commit b611746 (PR #1090) when exports were changed from wildcard
  to explicit. This restores the export for users who need NullAnalytics
  in their test code.

Author: laksh-twilio

packages/browser/src/index.ts

@@ -1,4 +1,4 @@
-export { Analytics } from './core/analytics'
+export { Analytics, NullAnalytics } from './core/analytics'
 export { AnalyticsBrowser } from './browser'
 export type {
   AnalyticsBrowserSettings,