Fix DRM failures for custom content in the demo

PR #1217 broke the demo app for custom clear content.  It caused DRM
errors by setting blank license server URIs in the config for custom
assets.  The fix is not to set license servers if the URI field is
blank.

Change-Id: Ia11ff5f49d246eeb5f17a18bd5ca4a4670a7e106

demo/asset_section.js

@@ -203,11 +203,14 @@ shakaDemo.preparePlayer_ = function(asset) {
 
   if (!asset) {
     // Use the custom fields.
+    var licenseServerUri = document.getElementById('licenseServerInput').value;
     var licenseServers = {};
-    commonDrmSystems.forEach(function(system) {
-      licenseServers[system] =
-          document.getElementById('licenseServerInput').value;
-    });
+    if (licenseServerUri) {
+      commonDrmSystems.forEach(function(system) {
+        licenseServers[system] = licenseServerUri;
+      });
+    }
+
     asset = /** @type {shakaAssets.AssetInfo} */ ({
       manifestUri: document.getElementById('manifestInput').value,
       // Use the custom license server for all key systems.