Fix warning in main.php for PHP 8+ (#1014)

* Fix PHP Warning in main.php for PHP 8+


```
PHP Warning:  Undefined array key "REMOTE_USER" in /usr/local/nagios/share/main.php on line 29
```

* Update html/main.php

props @tsadpbb

Co-authored-by: tsadpbb <90269351+tsadpbb@users.noreply.github.com>

* Update main.php

Missed a parenthesis. :)

---------

Co-authored-by: tsadpbb <90269351+tsadpbb@users.noreply.github.com>

Author: aidvu

html/main.php

@@ -26,7 +26,7 @@
 						"Click here to watch the entire Nagios Core 4 Tour!</a>";
 	<?php } ?>
 	$(document).ready(function() {
-		var user = "<?php echo htmlspecialchars($_SERVER['REMOTE_USER']); ?>";
+		var user = "<?php echo htmlspecialchars($_SERVER['REMOTE_USER'] ?? ''); ?>";
 
 		<?php if ($cfg["enable_page_tour"]) { ?>
 			vBoxId += ";" + user;