default missing ind1 and ind2 to blank

Should be fully backward compatible.

I am given library data which contains records like

```
<marc:datafield tag="946" ind1=" " ind2=" ">
    <marc:subfield code="a">No</marc:subfield>
  </marc:datafield>
  <marc:datafield tag="949">
    <marc:subfield code="a">Article</marc:subfield>
  </marc:datafield>
```

(missing `ind1` & `ind2` in 949), which makes `map_xml()` parsing fail with `KeyError: (None, u'ind1')`.