[BUGFIX] #15564 Generated admin API token expires immediately

When admin token lifetime setting is empty, the token will expire
immediatly

Author: krukas

app/code/Magento/Webapi/Model/Authorization/TokenUserContext.php

@@ -133,6 +133,11 @@ private function isTokenExpired(Token $token): bool
             // other user-type tokens are considered always valid
             return false;
         }
+
+        if (empty($tokenTtl)) {
+        	return false;
+        }
+
         if ($this->dateTime->strToTime($token->getCreatedAt()) < ($this->date->gmtTimestamp() - $tokenTtl * 3600)) {
             return true;
         }