Fix set $content optionnal

[Olwyne]

This fix the following fatal error appearing on all pages of the plugin :

Fatal error: Uncaught TypeError: WP2Static\WordPressAdmin::wp2staticAdminFooterText(): Argument #1 ($content) must be of type string, null given, called in /var/www/html/wp-includes/class-wp-hook.php on line 308 and defined in /var/www/html/wp-content/plugins/wp2static/src/WordPressAdmin.php:427 Stack trace: #0 /var/www/html/wp-includes/class-wp-hook.php(308): WP2Static\WordPressAdmin::wp2staticAdminFooterText(NULL) #1 /var/www/html/wp-includes/plugin.php(205): WP_Hook->apply_filters(NULL, Array) #2 /var/www/html/wp-admin/admin-footer.php(48): apply_filters('admin_footer_te...', '<span id="foote...') #3 /var/www/html/wp-admin/admin.php(297): require_once('/var/www/html/w...') #4 {main} thrown in /var/www/html/wp-content/plugins/wp2static/src/WordPressAdmin.php on line 427

[stellarpower]

Thank you for this!!