title: Query Mongodb docs with the dict field not exists or exists and empty
body: <p>Assume there is a some docs:</p>
  
  <pre><code># No 'value'
  {
  'name': 'T1',
  }
  
  # 'value' is a non-empty dict
  {
  'name': 'T1',
  'value': {'a':'A', 'b':'B'}
  }
  
  # 'value' is a empty dict
  {
  'name': 'T1',
  'value' : {}
  }
  </code></pre>
  
  <p>What I want to query is docs with <code>value</code> not exists or <code>value</code> exists and is empty. I tried following method and doesn't work:</p>
  
  <pre><code>cursor = collection.find({
              'name': {"$exists": 1},
              'value': {"$or": [{"$exists":0}, {"$eq": {}}]}
          })
  </code></pre>
  
  <p>The error:</p>
  
  <pre><code>pymongo.errors.OperationFailure: unknown operator: $or
  </code></pre>