Don't crash on empty enum declarations

Fixes #281

MOE_MIGRATED_REVID=197211835

Author: cushon

core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

@@ -834,10 +834,24 @@ public boolean visitEnumDeclaration(ClassTree node) {
       members.add(member);
     }
     if (enumConstants.isEmpty() && members.isEmpty()) {
-      builder.open(ZERO);
-      builder.blankLineWanted(BlankLineWanted.NO);
-      token("}");
-      builder.close();
+      if (builder.peekToken().equals(Optional.of(";"))) {
+        builder.open(plusTwo);
+        builder.forcedBreak();
+        token(";");
+        builder.forcedBreak();
+        dropEmptyDeclarations();
+        builder.close();
+        builder.open(ZERO);
+        builder.forcedBreak();
+        builder.blankLineWanted(BlankLineWanted.NO);
+        token("}", plusTwo);
+        builder.close();
+      } else {
+        builder.open(ZERO);
+        builder.blankLineWanted(BlankLineWanted.NO);
+        token("}");
+        builder.close();
+      }
     } else {
       builder.open(plusTwo);
       builder.blankLineWanted(BlankLineWanted.NO);

core/src/test/resources/com/google/googlejavaformat/java/testdata/i281.input

@@ -0,0 +1,8 @@
+public enum Empty {}
+public enum Empty {;}
+public enum Empty {;;;}
+public enum Empty {; // comment
+;;}
+public enum Empty {;
+;; // comment
+}

core/src/test/resources/com/google/googlejavaformat/java/testdata/i281.output

@@ -0,0 +1,20 @@
+public enum Empty {}
+
+public enum Empty {
+  ;
+}
+
+public enum Empty {
+  ;
+  ;;
+}
+
+public enum Empty {
+  ; // comment
+  ;;
+}
+
+public enum Empty {
+  ;
+  ;; // comment
+}