fix(replace): do not transform code if no values are given (#17)

* replace: do not transfrom code is no values are given

Fixes #16

* add test

* npm test

packages/replace/src/index.js

@@ -51,6 +51,7 @@ export default function replace(options = {}) {
     name: 'replace',
 
     transform(code, id) {
+      if (!keys.length) return null;
       if (!filter(id)) return null;
 
       const magicString = new MagicString(code);

packages/replace/test/fixtures/form/replace-nothing/_config.js

@@ -0,0 +1,4 @@
+module.exports = {
+  description: 'replaces nothing',
+  options: { }
+};

packages/replace/test/fixtures/form/replace-nothing/input.js

@@ -0,0 +1 @@
+console.log('as-it'); // eslint-disable-line

packages/replace/test/fixtures/form/replace-nothing/output.js

@@ -0,0 +1 @@
+console.log('as-it');

packages/replace/test/snapshots/form.js.md

@@ -52,3 +52,9 @@ Generated by [AVA](https://ava.li).
     `const one = 1; // eslint-disable-line␊
     ␊
     console.log(one);`
+
+## replace-nothing: replaces nothing
+
+> Snapshot 1
+
+    'console.log(\'as-it\'); // eslint-disable-line'