bug #503 [Examples] Fix env() function to check for empty string values (OskarStark)

This PR was merged into the main branch.

Discussion
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[Examples] Fix `env()` function to check for empty string values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Docs?         | no
| Issues        | --
| License       | MIT

The env() function now checks both isset() and empty string to properly handle environment variables defined in .env but with empty values.

Commits
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ee068ab Fix env() function to check for empty string values

Author: chr-hertel

examples/bootstrap.php

@@ -25,7 +25,7 @@
 
 function env(string $var)
 {
-    if (!isset($_SERVER[$var])) {
+    if (!isset($_SERVER[$var]) || '' === $_SERVER[$var]) {
         printf('Please set the "%s" environment variable to run this example.', $var);
         exit(1);
     }