[jQuery] Empty

Hi,

I have the following on a JQuery Code:

   $('<li class="DynamicList"></li>').appendTo('#Themes')
   .append(subject)
   .append('<br />')
   .append(levels)
   .append('<br />')

How can I add

   .append(subject)
   .append('<br />')

only if subject is not empty and

   .append(levels)
   .append('<br />')

only if levels is not empty?

Levels and Subject are declared as: var subject = [], levels = [];

Thanks,
Miguel