Improve verification if env variable is defined

Dotenv.php

@@ -86,12 +86,11 @@ final class Dotenv {
   public function receiveGlobal($key, $prompt, $default = '') {
     $key = strtoupper($key);
     $value = getenv($key);
-    if (empty($value)) {
+    if ($value === FALSE) {
       if ($this->io->isInteractive()) {
         $value = $this->io->ask($prompt . ': ');
       }
-      /** @noinspection NotOptimalIfConditionsInspection */
-      if (empty($value)) {
+      if ($value === FALSE) {
         $value = $default;
       }
       $this->put($key, $value, TRUE);