pass empty string instead of null to prevent NPE

[OnkelDok]

Because some commits ago we introduced the null checks (

portfolio/name.abuchen.portfolio/src/name/abuchen/portfolio/model/ConfigurationSet.java

Lines 64 to 69 in f47258a

	public Configuration(String uuid, String name, String data)
	{
	this.uuid = Objects.requireNonNull(uuid);
	this.name = Objects.requireNonNull(name);
	this.data = Objects.requireNonNull(data);
	}

) we now have to pass empty string instead of null.

In current version (with passing null) this leads to errors when creating an empty new portfolio file. You cannot save the new file. The "Save" or "Save as" menu entries are disabled.