#6 Fix missing user input value

Dotenv.php

@@ -99,7 +99,7 @@ final class Dotenv {
       if (!empty($prompt) && $this->io->isInteractive()) {
         $value = $this->io->ask($prompt . ': ');
       }
-      if ($value === FALSE) {
+      if ($value === FALSE || $value === NULL) {
         $value = $default;
       }
       $this->put($key, $value, TRUE);