Avoid custom headers to be undefined

lib/api/map/preview-template-controller.js

@@ -353,7 +353,9 @@ function incrementMapViews ({ metadataBackend }) {
 
             const statTag = mapConfig.obj().stat_tag;
 
-            res.set('Carto-Stat-Tag', `${statTag}`);
+            if (statTag) {
+                res.set('Carto-Stat-Tag', `${statTag}`);
+            }
 
             metadataBackend.incMapviewCount(user, statTag, (err) => {
                 if (err) {

lib/api/middlewares/increment-map-view-count.js

@@ -5,7 +5,9 @@ module.exports = function incrementMapViewCount (metadataBackend) {
         const { mapConfig, user } = res.locals;
         const statTag = mapConfig.obj().stat_tag;
 
-        res.set('Carto-Stat-Tag', `${statTag}`);
+        if (statTag) {
+            res.set('Carto-Stat-Tag', `${statTag}`);
+        }
 
         // Error won't blow up, just be logged.
         metadataBackend.incMapviewCount(user, statTag, (err) => {