fix(tests): drop unused ts-jest transform from shared jest preset

[deejayem]

nxPreset registers a ts-jest transform, but every project in this workspace overrides it with its own @swc/jest transform, so the entry is resolved by jest at startup yet never executed. ts-jest is not a dependency of any of these projects — it is only pulled in transitively by one example's devDependencies — so any install that does not include that example (e.g. pnpm install --filter "cdktn-cli...") makes every test run fail at config load with "Module ts-jest in the transform option was not found".

Override the inherited transform with an empty map; each project supplies its own.

Related issue

Fixes #381

Description

Make the jest.preset.js transform an empty map.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas - n/a, the change is too small to need it
• I have made corresponding changes to the documentation if applicable - n/a, none needed
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable - n/a, this is tested by existing tests passing when using --filter
• New and existing unit tests pass locally with my changes