Fix issue when $magickBinaryPath is empty

Author: Pierstoval

src/Command.php

@@ -117,7 +117,7 @@ public static function findMagickBinaryPath(?string $magickBinaryPath): string
             throw new MagickBinaryNotFoundException($magickBinaryPath);
         }
 
-        if (!\is_executable($magickBinaryPath)) {
+        if ($magickBinaryPath && !\is_executable($magickBinaryPath)) {
             throw new \InvalidArgumentException(\sprintf('The specified script (%s) is not executable.', $magickBinaryPath));
         }