[10.x] Address Null Parameter Deprecations in UrlGenerator (#51148)

* Replace null values with empty strings for strpos.
Replace null values with empty strings for preg_match.

* Revert change to UrlGenerator.
Change default header get to empty string.

Author: aldobarr

src/Illuminate/Testing/TestResponse.php

@@ -310,7 +310,7 @@ public function assertHeaderMissing($headerName)
     public function assertLocation($uri)
     {
         PHPUnit::assertEquals(
-            app('url')->to($uri), app('url')->to($this->headers->get('Location'))
+            app('url')->to($uri), app('url')->to($this->headers->get('Location', ''))
         );
 
         return $this;
@@ -324,7 +324,7 @@ public function assertLocation($uri)
      */
     public function assertDownload($filename = null)
     {
-        $contentDisposition = explode(';', $this->headers->get('content-disposition'));
+        $contentDisposition = explode(';', $this->headers->get('content-disposition', ''));
 
         if (trim($contentDisposition[0]) !== 'attachment') {
             PHPUnit::fail(