Do not set domainRegExp for local files

`parse_url($this->url, \PHP_URL_HOST)` will return `null` for local filesystem path.
Casting it to `string` will produce an empty regular expression,
which would match any link when computing link density.

(cherry picked from commit c7208f6ad2)

This also fixes a warning since 1.x passes the `null` directly to `preg_replace` instead of explicitly casting it to `string`.

src/Readability.php

@@ -1396,7 +1396,10 @@ class Readability implements LoggerAwareInterface
         $this->logger->debug('Parsing URL: ' . $this->url);
 
         if ($this->url) {
-            $this->domainRegExp = '/' . strtr(preg_replace('/www\d*\./', '', parse_url($this->url, \PHP_URL_HOST)), ['.' => '\.']) . '/';
+            $host = parse_url($this->url, \PHP_URL_HOST);
+            if (null !== $host) {
+                $this->domainRegExp = '/' . strtr(preg_replace('/www\d*\./', '', $host), ['.' => '\.']) . '/';
+            }
         }
 
         mb_internal_encoding('UTF-8');