Add check for existence of $_SERVER['SERVER_SOFTWARE'] (#31)

index.php

@@ -200,7 +200,7 @@ class OpCacheService
                 ]
             ]
         );
-        
+
         if (!empty($status['interned_strings_usage'])) {
             $overview['readable']['interned'] = [
                 'buffer_size' => $this->size($status['interned_strings_usage']['buffer_size']),
@@ -220,7 +220,7 @@ class OpCacheService
             $config['version'],
             [
                 'php'    => phpversion(),
-                'server' => $_SERVER['SERVER_SOFTWARE'],
+                'server' => empty($_SERVER['SERVER_SOFTWARE']) ? '' : $_SERVER['SERVER_SOFTWARE'],
                 'host'   => (function_exists('gethostname')
                     ? gethostname()
                     : (php_uname('n')