Empty text node raises 'NoneType' object has no attribute 'encode'

In splunklib/results.py at line 257 `values.append(elem.text.encode('utf8'))` can raise an AttributeError for encode.

If the xml looks like:

```
<field k='named_field'>
    <value><text></text></value>
</field>
```

elem.text will be the None object, not an empty string as one might expect. I've used this instead:

```
t = elem.text
if not t:
    t = ''
 values.append(t.encode('utf8'))
```
