Fix bug 62218 / SMWSparqlResultParser / "InvalidArgumentException" / $datatype needs to be a string

```
Unexpected non-MediaWiki exception encountered, of type "InvalidArgumentException"
[a858bc47] /fuseki/index.php?title=Main_Page&action=submit Exception from
line 253 of ...\SemanticMediaWiki\includes\export\SMW_Exp_Element.php: $datatype needs to be a string
Backtrace:
#0 ...\SemanticMediaWiki\includes\sparql\SMW_SparqlResultParser.php(146): SMWExpLiteral->__construct(string, boolean)
#1 [internal function]: SMWSparqlResultParser->xmlHandleCData(resource, string)
```

- https://gerrit.wikimedia.org/r/116760
- https://bugzilla.wikimedia.org/show_bug.cgi?id=62218

Relates to #337 
can be private