Do not check preconditions on REST parent resources

If 'If-None-Match: *' is specified this should only fail if the
specified REST resource already exists and not if the parent resource
exists.

Change-Id: I299ff4cdcd4d932401507eaca4f5807718be6dcd
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-httpd/src/main/java/com/google/gerrit/httpd/restapi/RestApiServlet.java

@@ -215,7 +215,9 @@ public class RestApiServlet extends HttpServlet {
         IdString id = path.remove(0);
         try {
           rsrc = rc.parse(rsrc, id);
-          checkPreconditions(req, rsrc);
+          if (path.isEmpty()) {
+            checkPreconditions(req, rsrc);
+          }
         } catch (ResourceNotFoundException e) {
           if (rc instanceof AcceptsCreate
               && path.isEmpty()