Handle empty API keys with no-op client

[marandaneto]

Summary

• initialize PostHog with a disabled no-op consumer when the API key is missing, null, empty, or trim-empty
• avoid sending capture/flag requests with an empty api_key
• add focused coverage for PostHog::init() and direct Client construction

Tests

• ./vendor/bin/phpunit --no-coverage --filter 'testInitWithoutApiKeyConfiguresNoOpClient|testInitWithEmptyApiKeyConfiguresNoOpClient|testInitWithWhitespaceApiKeyConfiguresNoOpClient|testClientWithNullApiKeyDoesNotSendRequests|testClientWithTrimEmptyApiKeyDoesNotSendRequests' test/PostHogTest.php\n- ./vendor/bin/phpunit --no-coverage (completed with existing warnings/deprecations)