Set jwtToken to null instead of blank string

[paryank]

getUsername() and getExpiration() throws error, when AccessToken is blank. In constructor, jwtToken should be set to null to avoid this error.

Issue #, if available:

Description of changes:

• Updated constructor() in src/CognitoAccessToken.js to set jwtToken property to null instead of empty string when AccessToken is not available.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.