Fix empty URL handling.

doc/changelog.txt

@@ -10,6 +10,8 @@ Changes:
   by default since they require third party modules.
 - plugins: Print a warning for enabled plugins that could not
   import needed third party modules.
+- checking: Treat empty URLs as same as parent URL.
+  Closes: GH bug #524
 
 Fixes:
 - checking: Catch XML parse errors in sitemap XML files and print them

linkcheck/checker/urlbase.py

@@ -315,8 +315,7 @@ class UrlBase (object):
         """
         log.debug(LOG_CHECK, "checking syntax")
         if self.base_url is None:
-            self.set_result(_("URL is missing"), valid=False)
-            return
+            self.base_url = u""
         if not (self.base_url or self.parent_url):
             self.set_result(_("URL is empty"), valid=False)
             return

tests/checker/data/misc.html.result

@@ -5,10 +5,9 @@ name %(datadir)s/misc.html
 valid
 
 url 
-cache key None
-real url 
-info The URL is outside of the domain filter, checked only syntax.
-error
+cache key file://%(curdir)s/%(datadir)s/misc.html
+real url file://%(curdir)s/%(datadir)s/misc.html
+valid
 
 url http://www.example.com/
 cache key http://www.example.com/