Fix --debug-check (#1916)

This got broken with the cursor commit.

cc @josephfrazier, would be nice to add some end to end test here :)

bin/prettier.js

@@ -201,7 +201,7 @@ function format(input, opt) {
           diff(input, pp);
       }
     }
-    return;
+    return {};
   }
 
   return prettier.formatWithCursor(input, opt);