DefaultCookieSerializer ignores null Cookie value

Previously a null Cookie value was returned by DefaultCookieSerializer
readCookieValues(). This could cause NullPointerExeptions later on.

This commit ignores cookies with a null value.

Fixes gh-392

spring-session/src/main/java/org/springframework/session/web/http/DefaultCookieSerializer.java

@@ -59,6 +59,9 @@ public class DefaultCookieSerializer implements CookieSerializer {
 			for (Cookie cookie : cookies) {
 				if (cookieName.equals(cookie.getName())) {
 					String sessionId = cookie.getValue();
+					if(sessionId == null) {
+						continue;
+					}
 					if(jvmRoute != null && sessionId.endsWith(jvmRoute)) {
 						sessionId = sessionId.substring(0, sessionId.length() - jvmRoute.length());
 					}