Add #toString to EmptyConfiguredTarget and ValueOrException$ValueOrExceptionValueImpl to help diagnose bug.

PiperOrigin-RevId: 238919901
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/EmptyConfiguredTarget.java b/src/main/java/com/google/devtools/build/lib/analysis/EmptyConfiguredTarget.java
index 90ef920..eb33eb1 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/EmptyConfiguredTarget.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/EmptyConfiguredTarget.java
@@ -39,4 +39,9 @@
   protected Object rawGetSkylarkProvider(String providerKey) {
     return null;
   }
+
+  @Override
+  public String toString() {
+    return "Empty" + super.toString();
+  }
 }