The code was not robust anymore in weird cases where file['k']=='', it is now.

``file['k'] == ''`` seems to happens in weird cases.

I got it uploading a file using account A in a shared folders detained by account B (using mega.py). There was no problem with the list of files in account A, but the file in the list of files of acount B had a ``file['k'] == ''``. The file was not visible with account B in the web interface (and mega.py was crashing to this bug).

Once account A downloaded the file (in the web interface), the file was visible with account B, and ``file[k] != ''``