Do not break when a dialog node's context has an empty key

[xverges]

If the dialog contains an empty line in a node's context editor

the node's context includes a "":"" dictionary element:

"parent": "Welcome",
            "context": {
                "": "",
                "clientProfile": {
...
                    "country": "Canada",
                    "firstName": "Guest"
                }
            },
...

and
dialog_json2xml.py crashes without giving the user any clue about how to fix the problem (ValueError: Empty tag name)