Fix #18192 -- Check return value for None

When IOError happens in django/utils/translation/trans_real.py function _translation() [line 128], the return value is None. After, django breaks with error message:
AttributeError: 'NoneType' object has no attribute '_info'
The None value must be replaced with fallback/NullTranslations (done with the next "if" statement [line 168]).