fix: restore the missing-SDK-key guard in index.js

[devin-ai-integration]

Summary

Routine SDK audit of this example; no version change was needed, but the "please set your SDK key" guard was dead code.

• @launchdarkly/node-server-sdk needs no bump: manifest declares ^9.12.1, latest on npm is 9.13.1, and there is no committed lock file, so npm install already resolves to latest 9.x.
• Fixed the unreachable guard — the placeholder default made if (!sdkKey) always false:

-const sdkKey = process.env.LAUNCHDARKLY_SDK_KEY ?? 'your-sdk-key';
+const sdkKey = process.env.LAUNCHDARKLY_SDK_KEY ?? '';

• Guard message now names the env var the README tells users to set, instead of pointing at index.js only.
• Titled fix: (not chore:) because it touches shipped example code.
• No deprecated APIs: init, waitForInitialization({timeout}), variation, on('update:...') are all current; no deprecation warnings under node --trace-deprecation.

Implementation details

Before this change, running node index.js without LAUNCHDARKLY_SDK_KEY initialized the SDK with the literal string your-sdk-key and failed with an opaque error, and exited 0 in the CI path:

error: [LaunchDarkly] Authentication failed. Double check your SDK key.
*** SDK failed to initialize: Error: Authentication failed. Double check your SDK key.

After:

$ node index.js
*** Please set LAUNCHDARKLY_SDK_KEY to your LaunchDarkly SDK key, or edit index.js to set sdkKey.
(exit 1)

Validation with a real key (hello-apps project, flag hello-boolean):

$ npm install                # installs @launchdarkly/node-server-sdk 9.13.1, 0 vulnerabilities
$ CI=1 LAUNCHDARKLY_SDK_KEY=<sdk key> LAUNCHDARKLY_FLAG_KEY=hello-boolean node index.js
info: [LaunchDarkly] Opened LaunchDarkly stream connection
*** SDK successfully initialized!
*** The 'hello-boolean' feature flag evaluates to true.
<banner>

README needed no change: it already documents LAUNCHDARKLY_SDK_KEY / LAUNCHDARKLY_FLAG_KEY and mentions no SDK version. Console app, so no screenshots. Diff is 2 insertions / 2 deletions.

Link to Devin session: https://app.devin.ai/sessions/3b513691f831464c8b12d98f09d0387f

[devin-ai-integration]

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[devin-ai-integration]

@cursor review