Also ok to be nullable if it comes from a parameter.

Sigh.
I'd blacklist the bad ones but they're a protected class.

If this causes any more trouble I'll revamp how nullable() works
and make it a method on Property that by default checks the annotation,
but that could be overridden. Then, I'd convert alllllll the places
that currently check the annotation...
Did the right thing and it's better.

Added nullable() to Property, which checks the annotation by default
but is overrideable.

In ConfigReferencePanelArrayListEditor, override the sub-property to
return false for nullable(), matching what the validator checks.