group by shows empty groups for falsey values

[Vincent Vinet]

In openerp/osv/orm.py

the method BaseModel.read_group checks if the group_by value is falsey to (I suppose) avoid returning None.
Unfortunately, this breaks grouping by values that can be falsey even when defined (ints, strings, floats?)

The resulting behavior is that you will see a group with the correct number of values, yet when you try to expand it, you get no results (or wrong results, if some values did have the value false)

As seen from the jsonrpc side, you will receive something along the lines of
{"result": [
    { "foo_count": 15,
      "__context": {...},
      "__domain": [["foo", "=", false]],
      "foo": 0}
], ...}

Now, attempting to expand the group will search with ("foo", "=", false) and not match any of the results that have 0.