Fixed deprecation warnings about passing null as parameter

Author: derrabus

AbstractUriElement.php

@@ -47,7 +47,7 @@ public function __construct(\DOMElement $node, string $currentUri = null, ?strin
         $this->currentUri = $currentUri;
 
         $elementUriIsRelative = null === parse_url(trim($this->getRawUri()), \PHP_URL_SCHEME);
-        $baseUriIsAbsolute = \in_array(strtolower(substr($this->currentUri, 0, 4)), ['http', 'file']);
+        $baseUriIsAbsolute = null !== $this->currentUri && \in_array(strtolower(substr($this->currentUri, 0, 4)), ['http', 'file']);
         if ($elementUriIsRelative && !$baseUriIsAbsolute) {
             throw new \InvalidArgumentException(sprintf('The URL of the element is relative, so you must define its base URI passing an absolute URL to the constructor of the "%s" class ("%s" was passed).', __CLASS__, $this->currentUri));
         }