Fix site::unlink method signature and docblock

cli/Valet/Site.php

@@ -325,10 +325,10 @@ public function getSites(string $path, Collection $certs): Collection
     /**
      * Unlink the given symbolic link.
      *
-     * @param  string  $name
+     * @param  string|null  $name
      * @return string
      */
-    public function unlink(string $name): string
+    public function unlink(?string $name = null): string
     {
         $name = $this->getRealSiteName($name);