do not fail when no votes are given, and only the antrag exists

This could very well be the case if an external person sends an antrag, but no 
one from the vorstand has voted yet.

For example, I tried to import a single mail, which had "[antrag"] in the
subject. The debug output was:
```
--------------------------------------------------------------------------------
DEBUG in ditzy [ditzy.py:164]:
Creating new antrag   testantrag
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
DEBUG in ditzy [ditzy.py:177]:
New antrag id: 2
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
DEBUG in ditzy [ditzy.py:202]:
No vote found in msg <20140421213219.GA15761@yourhost>
--------------------------------------------------------------------------------
```