Fix #33968: fix NPE in UserAgentFilter

Author: LEDfan

src/main/java/eu/openanalytics/containerproxy/security/UserAgentFilter.java

@@ -33,7 +33,8 @@ public class UserAgentFilter extends OncePerRequestFilter {
 
     @Override
     protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws ServletException, IOException {
-        if (request.getHeader("User-Agent").contains("ms-office")) {
+        String header = request.getHeader("User-Agent");
+        if (header != null && header.contains("ms-office")) {
             // see #33103
             // send empty response if request made by MS Office
             // otherwise the user will not be able to sign-in (or get logged out)