fix(core): Fix null check in IAM logic

Author: lpatino10

core/src/main/java/com/ibm/watson/developer_cloud/service/security/IamTokenManager.java

@@ -162,7 +162,7 @@ private boolean isAccessTokenExpired() {
    * @returns whether the current managed refresh token is expired or not
    */
   private boolean isRefreshTokenExpired() {
-    if (tokenData.getExpiration() != null) {
+    if (tokenData.getExpiration() == null) {
       return true;
     }