Fix bug in recorder servlet

The servlet always returns 400 bad request, because of an inverted if
condition

server/src/main/java/ctbrec/recorder/server/RecorderServlet.java

@@ -75,7 +75,7 @@ public class RecorderServlet extends AbstractCtbrecServlet {
                     .build();
             JsonAdapter<Request> requestAdapter = moshi.adapter(Request.class);
             Request request = requestAdapter.fromJson(json);
-            if (request.action != null) {
+            if (request.action == null) {
                 sendError(resp, SC_BAD_REQUEST, "{\"status\": \"error\", \"msg\": \"action is missing\"}");
                 return;
             }