Reapply "Handle possibility of a null token."

This reverts commit cefb046.

CHANGES.md

@@ -13,6 +13,7 @@
 
 ##### Fixes :wrench:
 
+- Fixed a bug that prevented using the Token panel with a new Cesium ion Server that had never had its token set before.
 - Fixed a bug that could cause a crash in `CesiumIonSession` when the object was garbage collected or the AppDomain was unloaded while network requests were in progress.
 - Fixed a bug that could cause `CesiumFlyToController` to unexpectedly interrupt a flight.
 - glTF `doubleSided` materials are now supported. Previously, models using this property would appear inside-out.

Editor/SelectIonTokenWindow.cs

@@ -110,7 +110,7 @@ private void OnGUI()
                 // The source will be set to "UseExisting" later by RefreshTokens
                 // if the _specifiedToken is from the currently signed-in ion account.
                 this._source = IonTokenSource.Specify;
-                if (this._specifiedToken.Length == 0 && session.IsConnected())
+                if (string.IsNullOrEmpty(this._specifiedToken) && session.IsConnected())
                 {
                     this._source = IonTokenSource.Create;
                 }

Runtime/CesiumIonServer.cs

@@ -38,13 +38,13 @@ public class CesiumIonServer : ScriptableObject
         /// found by searching the logged-in Cesium ion account for the
         /// DefaultIonAccessToken.
         /// </summary>
-        public string defaultIonAccessTokenId;
+        public string defaultIonAccessTokenId = "";
 
         /// <summary>
         /// The default token used to access Cesium ion assets at runtime. This token
         /// is embedded in packaged games for use at runtime.
         /// </summary>
-        public string defaultIonAccessToken;
+        public string defaultIonAccessToken = "";
 
         // The server URL for which we are currently attempting to deduce an API URL. When this property
         // is null, we're not currently loading a config.json in order to deduce an API URL for the server.