test: support full-null relationship coverage

[elpete]

Closes #154

Issue review

Recommendation: 10/10. Empty belongs-to relationships must work under Lucee full-null support. Null mode is an engine-level compatibility setting, and relationship access should preserve the same semantic result in either representation.

Reasons for:

• A missing foreign key is normal relationship state, not an exceptional condition.
• Quick and qb should accept a real null anywhere the partial-null runtime previously supplied an empty string.
• Tests should assert Quick null semantics instead of coupling unrelated behavior to one engine representation.

Tradeoffs:

• Full-null and partial-null modes intentionally expose different raw values. Cross-mode tests need semantic assertions or narrowly normalized snapshots when their actual subject is casing or query shape.

Reproduction review

I launched a separate Lucee 6.2.8.20 server with -Dlucee.full.null.support=true and current qb 14.0.0-beta.3. The existing public test getInstance( "Post" ).find( 7777 ).getAuthor() returned null without throwing, so the original Quick/qb exception no longer reproduces.

The first full-null suite run exposed seven failing assertions and zero errors. They were stale tests that hard-coded partial-null empty strings or compared snapshots without normalizing null representation; the product behavior itself was correct. I updated those tests to assert public null semantics where possible and normalized only the representation fields in snapshot-focused tests.

Implementation

No production behavior change was necessary. The test updates:

• use isNullAttribute() and isNullValue() for entity null behavior;
• keep the BooleanCast regression entirely on the public entity flow and assert semantic null preservation;
• verify readonly persistence through a reloaded Quick entity;
• normalize nullable fields only in memento/asQuery snapshot comparisons;
• correct the expected component casing of externalID;
• make the belongs-to dissociation assertion valid in both null modes.

Validation

Lucee full-null mode:

• Focused affected bundles: 76 passed, 0 failed, 0 errors
• Full suite: 495 passed, 0 failed, 0 errors, 3 skipped

Normal Lucee mode:

• Focused affected bundles: 76 passed, 0 failed, 0 errors
• Full suite: 495 passed, 0 failed, 0 errors, 3 skipped

Also:

• box run-script format
• git diff --check

Dependency: latest prerelease qb 14.0.0-beta.3.

[elpete]

Attempted reproduction: I ran the existing public empty-belongs-to flow on a dedicated Lucee 6.2.8.20 server started with -Dlucee.full.null.support=true and qb 14.0.0-beta.3. getInstance( "Post" ).find( 7777 ).getAuthor() returned null and did not throw, so the original exception no longer reproduces. I then made the surrounding tests representation-independent and verified the entire suite in both full-null and normal modes: each finished with 495 passed, 0 failed, 0 errors, and 3 skipped.