Make StringConsumer.finished an empty default method.

Change-Id: I6f2c0ed84c93eb64c2ad42836708db18c38293c3
diff --git a/src/main/java/com/android/tools/r8/StringConsumer.java b/src/main/java/com/android/tools/r8/StringConsumer.java
index 5d02846..6e440eb 100644
--- a/src/main/java/com/android/tools/r8/StringConsumer.java
+++ b/src/main/java/com/android/tools/r8/StringConsumer.java
@@ -41,7 +41,7 @@
    *
    * @param handler Diagnostics handler for reporting.
    */
-  void finished(DiagnosticsHandler handler);
+  default void finished(DiagnosticsHandler handler) {}
 
   static EmptyConsumer emptyConsumer() {
     return EmptyConsumer.EMPTY_CONSUMER;