Allow LazyProperty to pretend that attributes are present

Lazy Properties unconditionally assign to their slot when first fetched, so
there's currently no way that this could ever be false.  Short-cut the
actual fetch for the time being; folk can specialize it in a LazyProperty
sub-class if they so choose.
Fix sense of bool(Record)

Good thing this hasn't gone out yet.  Tests should have caught this!
Don't neglect collection item NO_CHANGE Diff's with fuzzy match

The fuzzy match code can actually result in no net differences, even if the
items did not exactly match the first time around.  Check for and emit them
if options.unchanged is selected.
Don't let diff be fooled by 'empty' values

Diff should be able to see through an accessor returning None instead of
an exception; add the support.
Regression shaken out by other fixes

This change didn't make any sense, but after investigating other regressions it
seems to have righted itself.  Revert to the previous version.