empty string conditionText should be treated similar to null

ejabberd sends the following stanza when a user is kicked off from reaching max simultaneous connections:

```xml
<error xmlns='http://etherx.jabber.org/streams'>
    <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
    <text xmlns='urn:ietf:params:xml:ns:xmpp-streams' lang=''>Replaced by new connection</text>
</error>
```

StreamError does not parse this correctly due to conditionText == "" and throws IllegalArgumentException "'conflict' can not contain a condition text". Expected behavior is to treat empty string as no conditionText, or pass in null to StreamError's constructor, not "".

Tested on android sdk v21.