title: Android - Check if a collection is empty and/or doesn't exists in Firebase
body: <p>Is there any way to check is a <code>subcollection</code> exists and/or is <code>empty</code> (as in the collection exists but doesn't have documents inside it) ?</p>
  
  <p>I want to display a "no appointment found" message if there's no appointment.</p>
  
  <p><strong>PS</strong> <em>I am working with <code>QuerySnapshots</code></em></p>
  
  <p><strong>This is what I have</strong></p>
  
  <pre><code>colRef.whereEqualTo("type", "vaccination")
              .whereEqualTo("isDone", "false")
              .orderBy("timestamp").limit(1).get()
              .addOnCompleteListener(new OnCompleteListener&lt;QuerySnapshot&gt;() {
                  @Override
                  public void onComplete(@NonNull Task&lt;QuerySnapshot&gt; task) {
                      if (task.isSuccessful()) {
                          for (QueryDocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
                              appointment = document.toObject(Appointment.class); 
                              address.setText(appointment.getAddress());
                              dir.setText(appointment.getName());
                          }
  
                      }
                  }
              })
              .addOnFailureListener(new OnFailureListener() {
                  @Override
                  public void onFailure(@NonNull Exception e) {
                      Log.e(TAG, ""+e);
                  }
              });
  </code></pre>