fix: add pre-check on expireTime in AccessTokenManager

[hengyuss]

Fixes: #6841

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[Aias00]

This still leaves the scheduled refresh vulnerable to unchecked parsing failures. The new null check handles a missing expiredTime, but doLogin only catches IOException; for example a successful response with data: null makes tokenMap null before this check, and a response where expiredTime is encoded as a string passes the null check but throws at the (long) expiredTime cast. Since start() runs login() from scheduleWithFixedDelay, an uncaught runtime exception can still suppress future refresh attempts. Could you validate the login payload and return false for malformed responses, and/or guard the scheduled task so refresh continues after one bad response?