Allow empty projection dictionary in find queries

[EtiennePelletier]

The current code that verifies pymongo query projections does not replicate MongoDB shell's behavior when the projection is an empty dictionary.

> db.test.find({}, {})
{ "_id" : ObjectId("5cec99eb81f850232d377fe1"), "value" : 0 }
{ "_id" : ObjectId("5cec99ec81f850232d377fe2"), "value" : 1 }
{ "_id" : ObjectId("5cec99ec81f850232d377fe3"), "value" : 2 }

PyMongo verifies if the projection is not None, but before converting it to a proper dictionary with  helpers._fields_list_to_dict, any projection whose boolean value is False gets replaced with {"id": 1}. In the case of an empty dictionary, this is annoying, as it prevents creating a projection variable with an empty dictionary and optionally adding values to it.

>>> list(coll.find({}, {}))
[{'_id': ObjectId('5cec99eb81f850232d377fe1')},
 {'_id': ObjectId('5cec99ec81f850232d377fe2')},
 {'_id': ObjectId('5cec99ec81f850232d377fe3')}]

Simple problematic use case:

# In the scope of a request
projection = {}
if not args['verbose']:
    projection['out'] = 0
return list(coll.find({}, projection))

Not so elegant workaround to bypass this problem:

coll.find({}, projection or None)

🤹‍♂️ 🐍

[behackett]

I'm open to making this change, but since PyMongo has behaved this way for almost the entirety of its existence we should wait until PyMongo 4.0 and list it as a potentially backward breaking change.

[rathisekaran]

Tracking ticket - PYTHON-1853

[pooooodles]

We are planning to make this change in PYTHON-1853 for PyMongo 4.0.