Not all tokens will have an expiration date

Expiration date is not a required claim so we need to check for expiration date availability before trying to check if the token is expired. That will avoid this NPE:

```
java.lang.NullPointerException: null
    at org.visola.spring.security.tokenfilter.jwt.JwtTokenService.verifyToken(JwtTokenService.java:73) ~[spring-security-token-filter-1.0.jar:na]
```
