title: Magento: addAttributeToFilter to list Empty Multi Select Attribute
body: <p>For a Multi Select attribute, how can I filter a product collection to find products where the attribute has nothing selected? I have tried the following, but none of them worked:</p>
  
  <pre><code>$productCollection-&gt;addAttributeToFilter( 'multi_select_attribute', array( 'null' =&gt; true ) );
  $productCollection-&gt;addAttributeToFilter( 'multi_select_attribute', array( 'null' =&gt; '' ) );
  $productCollection-&gt;addAttributeToFilter( 'multi_select_attribute', array( 'eq' =&gt; '' ) ); 
  $productCollection-&gt;addAttributeToFilter( 'multi_select_attribute', array( 'finset' =&gt; '') );
  </code></pre>
  
  <p>All return an empty collection.</p>
  
  <p>So, how can I filter the product collection to find products where the 'multi_select_attribute' has nothing selected?</p>
  
  <p>EDIT - Additional Information:</p>
  
  <p>This is what I'm doing:</p>
  
  <pre><code>$productCollection = Mage::getResourceModel('catalog/product_collection');
  $this-&gt;_addProductAttributesAndPrices($productCollection);
  $productCollection-&gt;addStoreFilter();
  $productCollection-&gt;addAttributeToFilter( ... one of the above ... );
  </code></pre>