fix: preserve uninitialized BoxLang attributes

[elpete]

Summary

• avoid copying uninitialized BoxLang accessor values into an entity's tracked attribute data
• continue syncing explicitly tracked nulls and initialized accessor values
• add a BoxLang regression covering untouched accessor-backed columns

The broader native BoxLang metadata compatibility work from the original local commit is already represented on next; this ports the remaining attribute synchronization guard without duplicating that work.

Verification

• box testbox run runner=http://127.0.0.1:60309/tests/runner.cfm bundles=tests.specs.integration.BaseEntity.MetadataSpec (BoxLang 1.16, full null: 15 passed)
• box run-script format:check
• git diff --check

The exhaustive local BoxLang run was stopped at the requester's direction; the PR matrix will provide the full cross-engine coverage.