Fixed appearance settings subpage is not rendered

fix brave/brave-browser#19633

When "New Tab Page Branded Wallpapers" is turned on from brave://flags,
this can be happened. When this flag is off, ViewCounterService is not
initialized and ViewCounterService::InitializeWebUIDataSource() is not
called. So, useThemesSubPage_ should check value existance first.

browser/resources/settings/brave_appearance_page/brave_theme.js

@@ -78,6 +78,7 @@ Polymer({
   },
 
   useThemesSubPage_: function() {
-    return loadTimeData.getString('superReferralThemeName') !== "";
+    return loadTimeData.valueExists('superReferralThemeName') &&
+      loadTimeData.getString('superReferralThemeName') !== '';
   }
 });