fix(docker): emit literal null for string env vars set to "null"

[yogeshwaran-c]

Description

The Docker entrypoint translator emits the string "null" instead of the JS literal null whenever a string-typed env var is set to null (e.g. VALIDATOR_URL=null). Per docs/usage/configuration.md, null is the documented way to disable the validator badge:

validatorUrl | VALIDATOR_URL | String="https://validator.swagger.io/validator" OR null

Today, running

docker run -d -p 80:8080 -e URL=http://mySwaggerSpec -e VALIDATOR_URL=null swaggerapi/swagger-ui

injects

validatorUrl: "null",

into the swagger-initializer.js, which the validator badge component then treats as a relative URL and renders as a broken link.

This change makes the translator detect the literal string "null" for string-typed config variables and emit the JS null value instead, restoring the documented behaviour. The fix applies to every string-typed config variable (URL, VALIDATOR_URL, OAUTH2_REDIRECT_URL, FILTER, etc.), which matches what the issue reporter and follow-up commenters described.

Motivation and Context

Closes #5519. The bug was acknowledged by a maintainer in the original thread:

Thanks for the report. You indeed discovered a bug in the docker work, as it's likely that NULL is parsed as "NULL", so we'll have to get that fixed.

A follow-up comment confirmed it also breaks URL=null (and any other string variable that allows null), which the present fix addresses.

How Has This Been Tested?

• npm run test:unit -- test/unit/docker/translator.js — 17 tests pass (15 existing + 2 new).
• npx eslint --max-warnings 0 docker/configurator/translator.js test/unit/docker/translator.js — clean.
• Two new regression tests cover both VALIDATOR_URL=null and the more general case (URL=null, FILTER=null).

The translator is build-time configuration code (it edits the dist HTML before nginx serves it), so it does not have a Cypress/browser surface to exercise.

Checklist

• My change requires a change to the documentation. — No, the existing docs already promise this behaviour.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.