nginx-http-auth filter: match server_name = ""

As documented at
http://nginx.org/en/docs/http/server_names.html#miscellaneous_names "If
no server_name is defined in a server block then nginx uses the empty
name as the server name."  This regex change allows us to match error
output for such a configuration.

The log line added to the tests was lifted from our logs verbatim; it
did not match without the patched regex.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>