[FIX] Fix broken translation cache for absent translation of 'false'

Here is a mini bug report for this PR :
- install a fresh 8.0 db
- load the french locale
- enable the french on the admin account preferences
- install the expense module (or some other modules)
→ traceback :

  File "/home/dadafkas/anybox/buildout.8.0/parts/odoo/openerp/tools/convert.py", line 1046, in convert_csv_import
    raise Exception(_('Module loading %s failed: file %s could not be processed:\n %s') % (module, fname, warning_msg))
Exception: Module loading resource failed: file resource/security/ir.model.access.csv could not be processed:
 Line 3 : 'bool' object has no attribute 'lower'

This is due to the 'false' string apparently not having a translation, so the result of the _get_translation was : [False].
The fix consists in filtering the result to remove the False if any.