Allow --debug-check to work with stdin (#2013)

* Allow --debug-check to work with stdin

This makes it easier to quickly test formatting code snippets with e.g.

    pbpaste | prettier --debug-check

Before, the following would happen:

    $ pbpaste | prettier --debug-check
    stdin: TypeError: Invalid data, chunk must be a string or buffer, not undefined
        at WriteStream.Socket.write (net.js:693:11)
        at writeOutput (/Users/josephfrazier/workspace/prettier/bin/prettier.js:375:18)
        at getStdin.then.input (/Users/josephfrazier/workspace/prettier/bin/prettier.js:275:7)
        at <anonymous>
        at process._tickCallback (internal/process/next_tick.js:169:7)
    $

Now, it looks like this:

    $ pbpaste | prettier --debug-check
    (stdin)
    $

* Test that --debug-check still prints filenames

See https://github.com/prettier/prettier/pull/2013#discussion_r120499306

bin/prettier.js

@@ -202,7 +202,7 @@ function format(input, opt) {
           diff(input, pp);
       }
     }
-    return {};
+    return { formatted: opt.filepath || "(stdin)\n" };
   }
 
   return prettier.formatWithCursor(input, opt);
@@ -280,7 +280,7 @@ if (stdin) {
   });
 } else {
   eachFilename(filepatterns, filename => {
-    if (write || argv["debug-check"]) {
+    if (write) {
       // Don't use `console.log` here since we need to replace this line.
       process.stdout.write(filename);
     }
@@ -358,7 +358,6 @@ if (stdin) {
         }
       }
     } else if (argv["debug-check"]) {
-      process.stdout.write("\n");
       if (output) {
         console.log(output);
       } else {