Fix version string for unsupported installations

For unsupported installations, $version['commit'] may not be defined,
leading to a warning:

E_WARNING (2)

Undefined array key "commit"
1. classes/config.php(316): ttrss_error_handler(Undefined array key "commit", classes/config.php)
2. prefs.php(173): get_version_html()

classes/config.php

@@ -313,7 +313,7 @@ class Config {
 
 		return sprintf("<span title=\"%s\n%s\n%s\">%s</span>",
 			date("Y-m-d H:i:s", ($version['timestamp'] ?? 0)),
-				$version['commit'],
+				$version['commit'] ?? '',
 				$version['branch'] ?? '',
 				$version['version']);
 	}