Disable SDK when API key is blank

[marandaneto]

Summary

• Treat missing, empty, or whitespace-only api_key as disabled/no-op configuration
• Avoid building an API client or starting senders/source workers when disabled
• Make captures no-op when the SDK is disabled

Tests

• mix test test/posthog/config_test.exs test/posthog/application_config_test.exs test/posthog/supervisor_test.exs
• mix test
• mix format --check-formatted
• mix credo --strict