Don't add "None" to imagery_used tag when other imagery was used

(closes #5565)

modules/renderer/background.js

@@ -164,7 +164,12 @@ export function rendererBackground(context) {
             window.location.replace('#' + utilQsString(q, true));
         }
 
-        var imageryUsed = [b.imageryUsed()];
+        var imageryUsed = [];
+
+        var current = b.imageryUsed();
+        if (current) {
+            imageryUsed.push(current);
+        }
 
         _overlayLayers
             .filter(function (d) { return !d.source().isLocatorOverlay() && !d.source().isHidden(); })

modules/renderer/background_source.js

@@ -467,7 +467,7 @@ rendererBackgroundSource.None = function() {
 
 
     source.imageryUsed = function() {
-        return 'None';
+        return null;
     };
 
 

modules/ui/commit.js

@@ -92,7 +92,9 @@ export function uiCommit(context) {
         }
 
         tags = _clone(_changeset.tags);
-        tags.imagery_used = context.history().imageryUsed().join(';').substr(0, 255);
+
+        var imageryUsed = context.history().imageryUsed().join(';').substr(0, 255);
+        tags.imagery_used = imageryUsed || 'None';
         _changeset = _changeset.update({ tags: tags });
 
         var header = selection.selectAll('.header')