Bugfix: check symbol for NULL in binding json dump

When dumping a binding, as is done during the binding event, check
symbol for NULL. If it is, dump json null. This prevents a crash when
running a binding that was configured with bindcode.

fixes #1379
Testcases: rename StartXDummy to StartXServer

Rename the package StartXDummy to StartXServer in the testcases library
because XDummy is no longer used.

No logic changes.