allowed extensions fail safe if POSTPONE_IF_NO_SUBS enabled

@miigotu like this?

it outputs the string with single quotes

```
>>> allowed_extensions="nfo,sft"
>>> allowed_extensions += ',srt'
>>> allowed_extensions = {x.strip() for x in allowed_extensions.split(',') if x.strip()}
>>> print allowed_extensions
set(['nfo', 'sft', 'srt'])
>>> ','.join(allowed_extensions)
'nfo,sft,srt'
```