Don't treat an empty string as an invalid URL.

To me, this is a relative URL with a path of "". I.e., it refers to the
same document as wherever the URL appeared, like when people do
<form action="" enctype="multipart/form-data" method="POST">

nselib/url.lua

@@ -152,8 +152,6 @@ function parse(url, default)
     -- initialize default parameters
     local parsed = {}
     for i,v in base.pairs(default or parsed) do parsed[i] = v end
-    -- empty url is parsed to nil
-    if not url or url == "" then return nil, "invalid url" end
     -- remove whitespace
     -- url = string.gsub(url, "%s", "")
     -- get fragment