Fix node resolution on dbt Fusion (missing version kwarg resolves to '' instead of None)

[allllllllez]

Fix node resolution on dbt Fusion (missing version kwarg resolves to '' instead of None)

Fixes #225

Problem

Every dbt_unit_testing.test() fails on dbt Fusion with Node ... not found.. The graph
lookup in macros/utils.sql (model_node) is not at fault — graph is fully populated and
the target node matches correctly. The actual cause is that node.version ends up as ''
instead of None when no version/v kwarg is passed. has_value() treats '' as "a
version was requested", filters by version == '', and finds nothing since real unversioned
models have version == None.

Three call sites share the same fragile pattern, which relies on dbt Core's Python Jinja2
Undefined semantics for a missing kwargs key — a dependency Fusion's Jinja implementation
doesn't honor the same way:

{% set node_version = kwargs["version"] | default(kwargs["v"]) | default(none) %}

Change

Replace the bracket-index + default() chain with kwargs.get("x", kwargs.get("y", none)) in:

• macros/overrides.sql (ref() override — hit by every single test via ref_tested_model())
• macros/tests.sql (dbt_unit_testing.test())
• macros/mock_builders.sql (mock_ref())

No change to macros/utils.sql — the graph lookup itself was already correct.

Verification

Using this repo's integration-tests/ suite (31 dbt_unit_testing.test() cases), on branch
fix/fusion-kwargs-default-empty-string:

Engine	Before	After
dbt Fusion 2.0.0-preview.205 (Postgres, experimental adapter)	31/31 tests fail to compile (Node ..._v not found)	29/31 succeed at compile. Remaining 2 fail with [DbDriverFailed] HTTP error: io: No route to host — an unrelated connectivity limitation of Fusion's experimental postgres adapter, not a render/Jinja error
dbt Core 1.7.11 (existing test matrix, Postgres)	31/31 build + 31/31 test pass	Same — no regression
dbt Core 1.12.3 (latest available via pip install dbt-postgres==1.11.0, Postgres)	n/a	Full run-integration-tests.sh passes, no regression

Full reproduction/verification commands are in #225.