API directory listing should filter out files with size=None (MakeFile but no Upload)

[Roman Yepishev]

At the moment when a PUT is issued to create a file, a temporary node is returned with content_path for uploading the file.

The problem is these temporary nodes are also returned when doing a query on a directory with include_children=true.
These files have size: None and in case the implementation does not know about this quirk, that can cause the code to fail on an invalid size value. Moreover directly querying the temporary node results in 404 response.

These temporary files should not be listed when querying for the directory listing.

[Roman Yepishev (rye)]

Alternatively we might want to specify this case in the docs since I see that we are using make_file() only during the tests and rely on current implementation.