Make driveFolderId on Registry entity optional

This defaults to null, and leaving it to null now simply disables reserved terms
exporting, rather than throwing an error every time the action runs.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138763161

java/google/registry/model/registry/Registry.java

@@ -270,7 +270,11 @@ public class Registry extends ImmutableObject implements Buildable {
    */
   String tldUnicode;
 
-  /** Id of the folder in drive used to publish information for this TLD. */
+  /**
+   * Id of the folder in drive used to public (export) information for this TLD.
+   *
+   * <p>This is optional; if not configured, then information won't be exported for this TLD.
+   */
   String driveFolderId;
 
   /** The type of the TLD, whether it's real or for testing. */