title: Adding a key with an empty value to Guava Multimap
body: <p>I have a need to add a key to a Guava Multimap with an empty collection as the value.  How do I accomplish this?</p>
  
  <p>I tried this:</p>
  
  <pre><code>map.put( "my key", null );
  </code></pre>
  
  <p>but calling get() returns a list with one element, which is null.  I worked around this by doing the following:</p>
  
  <pre><code>map.putAll("my key2", new ArrayList())
  </code></pre>
  
  <p>but I'm wondering if this is a bad thing to do?  I know Guava automatically removes a key when the last value is removed to keep containsKey() consistent.  What's my best option here?</p>