title: $addFields when no $match found
body: <p>Iam new Mongodb developer i wrote mongodb aggregation.one of my field lampStatus : "OFF" is 30 recors are there iam using "{ $match: {lampStatus : "OFF"}}"i got 30 records but ' lampStatus : "OFF" 'there is no records iam getting Fetched 0 record(s) but how to get zero value in the above aggergation.</p>
  
  <pre><code>db.collection.aggregate([
  
                    { $match:{'type':'L'}},
                    { $match: {lampStatus : "ON"}},
                    { $group: { _id : null, TotalLights: { $sum: 1 } } },
                    { $project: { _id: 0, TotalLights: 1 } }
  
  
  
  
                ])
  </code></pre>
  
  <p>output:Fetched 0 record(s) in 0ms</p>
  
  <pre><code>expected outout:"TotalLights" : 0
  </code></pre>