Annotate DefaultCookieSerializer.setCookiePath as @nullable

The cookiePath field and javadoc already allow null (fall back to the
request context path). setSameSite already has @nullable; setCookiePath
was missing it.

Closes gh-3883

Signed-off-by: zanarelli <zanarelli.dev@gmail.com>

Author: zanarellidev

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

@@ -296,7 +296,7 @@ private boolean isSecureCookie(HttpServletRequest request) {
 	 * @param cookiePath the path of the Cookie. If null, the default of the context path
 	 * will be used.
 	 */
-	public void setCookiePath(String cookiePath) {
+	public void setCookiePath(@Nullable String cookiePath) {
 		this.cookiePath = cookiePath;
 	}