RFR RT-33113: HtmlEditor not printing

[Phil Race]

http://cr.openjdk.java.net/~prr/fxprinting/RT-33113/

https://javafx-jira.kenai.com/browse/RT-33113

The problem is that printing doesn't have a screen so we get a NPE.
We likely should make printing report a dummy screen so that callers
don't need to be aware of this, but at this stage in the game the safer
thing is for is to check for null and interpret that as the printing case.

-phil.