fix: no-op setup without api key

[marandaneto]

💡 Motivation and Context

Setup currently continued initializing after the API key trimmed to an empty string. Java callers could also pass a null Android API key and hit SDK init unexpectedly.

This changes null API keys to trim to an empty string and makes core/stateless setup return before initialization when the key is empty after trimming. Android setup skips Android-specific configuration for invalid keys.

💚 How did you test it?

• ./gradlew :posthog:test --tests "com.posthog.PostHogConfigTest" --tests "com.posthog.PostHogStatelessTest" --tests "com.posthog.PostHogTest"
• ./gradlew :posthog-android:testDebugUnitTest --tests "com.posthog.android.PostHogAndroidConfigTest" --tests "com.posthog.android.PostHogAndroidConfigJavaTest" --tests "com.posthog.android.PostHogAndroidTest"
• make checkFormat
• ./gradlew :posthog:apiCheck :posthog-android:apiCheck

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Added a changeset file for the affected packages
• Added the "release" label to the PR to indicate we're publishing new versions for the affected packages

[github-actions]

posthog-android Compliance Report

Date: 2026-04-30 10:42:49 UTC
Duration: 461ms

⚠️ Some Tests Failed

0/1 tests passed, 1 failed

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Feature_Flags Tests

⚠️ 0/1 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	278ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'