feat(Casts): delegate null handling to custom casts

[elpete]

Summary

• invoke configured custom casts for database null values by default
• preserve native nulls and configured null sentinels in BooleanCast
• safely support cast getters and setters that return native null
• add regression coverage with a custom null-aware cast

Fixes #275.

This incorporates the core direction discussed in #276 while adding BooleanCast compatibility and tests.

Test-first reproduction

Before the implementation, allows custom casts to handle null database values failed because the custom cast received no opportunity to transform the null value (expected casted-null, received an empty string). The test passes after the change.

The BooleanCast regression is covered through the normal Quick entity flow: load the nullable column with find(), read it through the entity null/accessor APIs, update an unrelated attribute, refresh, and verify the Boolean attribute remains null.

Verification

• qb@be resolved locally to 14.0.0-beta.3
• AttributeCastsSpec: 14 passed, 1 known unrelated qb 14 compatibility failure
• NullValuesSpec: 5 passed, 0 failed
• GitHub Actions: all 12 engine/ColdBox combinations passed, including Lucee 5/6, Adobe 2021/2023/2025, and BoxLang CFML
• GitHub Actions formatting and security checks passed
• box run-script format:check
• git diff --check