Fix 'allow unresolved' for at least two list cases

lists did not respond well to cases where only some of their
elements were resolved or even when none of them were resolved
but one of their object identities happened to change. Previously
this would throw a nasty 'bug or broken' exception so it seems
unlikely to have unintended side effects.
- it gets recomputed anyway to check for illegal state -- rightly so since this logic is broken.
- there is exactly one other caller, and it has a non-null parameter.

you can replace null with a ternary operator predicated on `contex.options().allowUnresolved()` if you like. I considered it but I knew it didn't matter and this method was both less intrusive and more in line with existing code. eg. this is what SimpleConfigObject does and the existing place to detect and complain about things that have failed to be resolved is in ResolveContext around line 116/132.