Missing default for configuration properties

[zbynek]

org.gwtproject.safecss.ForceCheckValidStyles is defined but never set which causes a NPE with GWT2 SuperDevMode; also it's never actually queried
com.google.gwt.safehtml.ForceCheckValidUri, com.google.gwt.safehtml.ForceCheckCompleteHtml are queried but never defined, which is technically OK, but not very transparent.

As a followup to gwtproject/gwt#9806 it would be nice to make sure all 3 properties are defined with "false" as a default and compared to "true" instead of null.

(I'll try to create a PR for this in the next few weeks)