html rewriter: cleanup rewrite_srcset, add more tests for empty rewrite

pywb/rewrite/html_rewriter.py

@@ -216,8 +216,7 @@ class HTMLRewriterMixin(object):
             return ''
 
         values = value.split(',')
-        values = map(lambda x: self._rewrite_url(x.strip()), values)
-        values = [v for v in values if v is not None]
+        values = [self._rewrite_url(v.strip()) for v in values]
         return ', '.join(values)
 
     def _rewrite_css(self, css_content):

pywb/rewrite/test/test_html_rewriter.py

@@ -49,6 +49,12 @@ r"""
 >>> parse('<base href="static/"/><img src="image.gif"/>', urlrewriter=no_base_canon_rewriter)
 <base href="static/"/><img src="/web/20131226101010im_/http://example.com/some/path/static/image.gif"/>
 
+# Empty url
+>>> parse('<base href="">')
+<base href="">
+
+>>> parse('<base href>')
+<base href>
 
 
 # HTML Entities