content: Don't update static content

pelican/contents.py

@@ -171,6 +171,9 @@ class Content(object):
         :param siteurl: siteurl which is locally generated by the writer in
                         case of RELATIVE_URLS.
         """
+        if not content:
+            return content
+
         hrefs = re.compile(r"""
             (?P<markup><\s*[^\>]*  # match tag with src and href attr
                 (?:href|src)\s*=)