title: in which case TreeBasedTable.create().rowMap().get(rowKey) will return an empty map
body: <p>In my project guava tables are used. version is 15.0. somehow in my logs the mapping for particular rowkey is coming empty like <code>{rowkey={}}</code> but I am not able to replicate it.
  I tried below approaches.</p>
  
  <pre><code>Table table = TreeBasedTable.create();
  table.put(rowkey, null,null)  // gives compilation error
  
  table.put(rowkey, "",null)    // giving compilation error
  
  table.put(rowkey, null,"")    // giving compilation error
  
  table.put(rowkey, "","")       // printing like {rowkey={=}}
  </code></pre>
  
  <p>Please help how can I get <code>{rowkey={}}</code>  if i print table.rowMap() 
  i.e map returning from <code>table.rowMap().get(rowKey)</code> is empty (not <code>null</code>).</p>