Don't return fatal error if CSS file is empty.

Fixes: #183

Author: cscott

src/util/TokenStreamBase.js

@@ -16,7 +16,7 @@ function TokenStreamBase(input, tokenData){
      * @property _reader
      * @private
      */
-    this._reader = input ? new StringReader(input.toString()) : null;
+    this._reader = new StringReader(input ? input.toString() : '');
 
     /**
      * Token object for the last consumed token.

tests/css/TokenStream.js

@@ -54,6 +54,14 @@
 
 
     //note: \r\n is normalized to just \n by StringReader
+    suite.add(new CSSTokenTestCase({
+        name : "Tests for empty input",
+
+        patterns: {
+            ""        : [CSSTokens.EOF]
+        }
+    }));
+
     suite.add(new CSSTokenTestCase({
         name : "Tests for Whitespace",