title: How to check if a string is empty in Emacs Lisp?
body: <p>I would like to write an if statement that will do something base on whether a string is empty.  For example:</p>
  
  <pre><code>(defun prepend-dot-if-not-empty (user-str)
     (interactive "s")
     (if (is-empty user-str)
       (setq user-str (concat "." user-str)))
     (message user-str))
  </code></pre>
  
  <p>In this contrived example, I'm using (is-empty) in place of the real elisp method.  What is the right way of doing this?</p>
  
  <p>Thanks</p>