Empty string (#152)

* test: add empty string test

* Fix empty strings

Author: goto-bus-stop

test/fixtures/prefix-empty-source.js

@@ -0,0 +1,3 @@
+const sf = require('sheetify')
+
+sf``

test/prefix.js

@@ -112,4 +112,31 @@ test('prefix', function (t) {
       }
     }
   })
+
+  t.test('should return a prefix when an empty string is passed', function (t) {
+    t.plan(1)
+
+    const ws = concat(function (buf) {
+      const res = String(buf).trim()
+      t.equal(res, '', 'css is equal')
+    })
+
+    const bOpts = { browserField: false }
+    const bpath = path.join(__dirname, 'fixtures/prefix-empty-source.js')
+    browserify(bpath, bOpts)
+      .transform(sheetify)
+      .transform(function (file) {
+        return through(function (buf, enc, next) {
+          const str = buf.toString('utf8')
+          this.push(str.replace(/sheetify\/insert/, 'insert-css'))
+          next()
+        })
+      })
+      .plugin('css-extract', { out: outFn })
+      .bundle()
+
+    function outFn () {
+      return ws
+    }
+  })
 })

transform.js

@@ -192,7 +192,7 @@ function transform (filename, options) {
   // then replace the AST nodes with new values
   // (obj, fn) -> null
   function iterate (val, done) {
-    if (val.css) return handleCss(val)
+    if (typeof val.css === 'string') return handleCss(val)
 
     if (/\.js$/.test(val.filename)) {
       delete require.cache[require.resolve(val.filename)]