AuthRequest: Fix Javadoc for return values

The methods' javadocs state that the return is 'null', but this was
changed some time ago to be Optional.empty().

Change-Id: I8f883cf7d79efb848d70043d23ecc707cfcdf1d0

java/com/google/gerrit/server/auth/AuthRequest.java

@@ -31,7 +31,7 @@ public abstract class AuthRequest {
   /**
    * Returns the username to be authenticated.
    *
-   * @return username for authentication or null for anonymous access.
+   * @return username for authentication or {@code empty} for anonymous access.
    */
   public final Optional<String> getUsername() {
     return username;
@@ -40,7 +40,7 @@ public abstract class AuthRequest {
   /**
    * Returns the user's credentials
    *
-   * @return user's credentials or null
+   * @return user's credentials or {@code empty}.
    */
   public final Optional<String> getPassword() {
     return password;