LPS-50700 force a NPE when source or target are null

When null is given as an argument to InputSource constructor, it fails
with a java.net.MalformedURLException, which is confusing (and doesn't
make any sense in our case).

Fail with NPE to make the API a little saner.
LPS-50700 unit tests for DiffHtml
LPS-50700 Use the right import
LPS-50700 Rename
LPS-50700 auto SF