transform() of XMLTools doesn't work if source doesn't have a systemID

[joeha480]

If no systemID is set for the source, no transform is performed. While it is a good idea to set a system id, the transform should still happen, or an exception should be thrown at least.

There is an if statement that requires a systemID to be set here:
https://github.com/brailleapps/dotify.common/blob/master/src/org/daisy/dotify/common/xml/XMLTools.java#L341

If this if-statement is not true, nothing happens. (Wow!)