title: Check if a string isn't nil or empty in Lua
body: <p>I've some currently some Lua code using the following syntax:</p>
  
  <pre><code>if (foo == nil or foo == '') then
      foo = "some default value"
  end
  </code></pre>
  
  <p>The goal of the if condition is to test foo is neither an empty string, neither a nil value.</p>
  
  <p>Can this code be simplified in one if test instead two?</p>