fix(eslint-plugin-turbo): Guard against missing tasks/pipeline in forEachTaskDef

[sleitor]

Summary

Fixes #12410

When turbo.json has neither a tasks nor a pipeline key, forEachTaskDef() calls Object.entries(config.pipeline) or Object.entries(config.tasks) on undefined, throwing a TypeError and crashing eslint-plugin-turbo at import time — before ESLint has linted a single file.

Fix

Add early-return null guards in forEachTaskDef() in packages/turbo-utils/src/get-turbo-configs.ts:

if ("pipeline" in config) {
  if (!config.pipeline) {
    return;
  }
  // ...
} else {
  if (!config.tasks) {
    return;
  }
  // ...
}

Tests

• Added a fixture __fixtures__/configs/empty-tasks/ with a minimal turbo.json that has no tasks or pipeline key.
• Added test cases to both the CJS and ESM no-undeclared-env-vars test suites verifying the plugin does not crash (and correctly flags undeclared env vars) when the config has no tasks defined.

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