Fix empty exports (#225)

```js
export {};
```

was printed as

```js
export
```

which is invalid.

This was discovered by #223

src/printer.js

@@ -1870,6 +1870,8 @@ function printExportDeclaration(path, options, print) {
     }
 
     parts.push(";");
+  } else {
+    parts.push("{};");
   }
 
   return concat(parts);

tests/export/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,6 @@
+exports[`test empty.js 1`] = `
+"export {};
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+export {};
+"
+`;

tests/export/empty.js

@@ -0,0 +1 @@
+export {};

tests/export/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname);