nbconvert: Remove cell from worksheet if null.

If someone wants to write a simple by-cell transformer that drops certain cells, the should be able to do something like the following:

    def transform_cell(self, cell, resources, index):
        if condition:
            return None, resources
        else:
            return cell, resources

This does not work in the current master.  Instead, the None gets set in the dict, then the template finds a None cell and either errors or inserts an `Unknown cell type` into the results.