Don’t fail when we have no known kanji.

I got 
```python
Traceback (most recent call last):
  File "/home/roland/anki/addons/addheisiginfo.py", line 354, in <lambda>
    browser.connect(c, SIGNAL("triggered()"), lambda e=browser: onHardestHeisigNumber(e))
  File "/home/roland/anki/addons/addheisiginfo.py", line 363, in onHardestHeisigNumber
    addMostDifficultHeisigNumber(browser.selectedNotes())
  File "/home/roland/anki/addons/addheisiginfo.py", line 191, in addMostDifficultHeisigNumber
    num = max((map(int, lookupKanjiInfo(srcTxt, 'heisig2010'))))
ValueError: max() arg is an empty sequence
```
when i tried to put the highest kanji of a field with no known kanji (non-Japanese text, kana, non-jōyō-kanji) into a field.
Just continuing when we have that case should fix that.

(To keep the pull request simple, i even left in your trailing white spaces, along with the crlf line endings, but you may want to take a look at [this branch](https://github.com/ospalh/worthy7s-anki-addons/tree/feature/clean-up). Afais the `.gitattributes` thing would let you keep the crlfs on Windows, while Unix uses should see pure lfs.)