Skip dummy subtest when EnvMatrix expands to empty

[denik]

Changes

When all EnvMatrix values are empty lists (e.g., DATABRICKS_BUNDLE_ENGINE = []), run the test directly without creating a #00 dummy subtest.

Before:

--- PASS: TestAccept/bundle/foo (0.00s)
    --- PASS: TestAccept/bundle/foo/#00 (1.23s)

After:

--- PASS: TestAccept/bundle/foo (1.23s)

This allows tests to opt out of the default engine matrix by setting DATABRICKS_BUNDLE_ENGINE = [] without creating a confusingly-named subtest.

Tests

Added selftests:

• selftest/envmatrix_empty — verifies no subtest is created when all EnvMatrix values are empty
• selftest/envmatrix_mixed — verifies empty vars are dropped while non-empty vars still create subtests

[github-actions]

Suggested reviewers

Based on git history of the changed files, these people are best suited to review:

• @shreyas-goenka -- recent work in acceptance/

Confidence: low

Eligible reviewers

Based on CODEOWNERS, these people or teams could also review:

@andrewnester, @anton-107, @pietern, @simonfaltum

_{Suggestions based on git history of 9 changed files (1 scored). See CODEOWNERS for path-specific ownership rules.}