fix(lambda): parse empty payload in lambda

[rhrlima]

Summary

Fixes Lambda Invoke crashing the AWS Node.js managed runtime with
SyntaxError: Unexpected end of JSON input when the caller sends an
empty body. The /2018-06-01/runtime/invocation/next handler in
RuntimeApiServer now writes "{}" for both null and zero-length
payloads, matching real AWS Lambda behaviour.

Addresses: #599

Type of change

• Bug fix (fix:)
• New feature (feat:)
• Breaking change (feat!: or fix!:)
• Docs / chore

AWS Compatibility

Incorrect behaviour (before this PR): Any aws lambda invoke (or
internal invoker) that handed floci an empty body caused the managed
Node.js runtime to receive a zero-length response from
/runtime/invocation/next. The runtime then ran JSON.parse("") and
crashed before the user's handler executed, returning
{"errorType":"SyntaxError","errorMessage":"Unexpected end of JSON input", ...}
with FunctionError: Unhandled.

Root cause: the /next handler defaulted to "{}" only when
invocation.getPayload() == null. JAX-RS materialises a missing request
body as new byte[0], so the path silently sent "" to the runtime
instead of valid JSON.

Behaviour after this PR matches real AWS Lambda: invoke without
--payload causes the handler's event argument to be {}; handlers
that don't read event succeed unconditionally.

This is a Node.js-runtime symptom, but the fix sits in the runtime API
server and therefore covers every managed runtime that does
JSON.parse on the event body. No SDK or wire-format changes; the
fixture verified against the AWS CLI v2 wire protocol.

Changes

File	Change
src/main/java/io/github/hectorvent/floci/services/lambda/runtime/RuntimeApiServer.java	The /next blockingHandler now treats null and zero-length payloads as "{}" so the runtime always receives valid JSON. Added an inline comment explaining the Node managed-runtime constraint that motivates this normalisation.
src/test/java/io/github/hectorvent/floci/services/lambda/runtime/RuntimeApiServerTest.java	New regression test nextEndpoint_emptyPayload_isDeliveredAsEmptyJsonObject enqueues a PendingInvocation whose payload is new byte[0], polls /next, and asserts the response body equals "{}". Without the fix, the assertion fails because the body is "".

Test plan

• ./mvnw test -Dtest=RuntimeApiServerTest passes (verified locally; all six existing cases plus the new regression).
• The new nextEndpoint_emptyPayload_isDeliveredAsEmptyJsonObject regression test fails before the fix (asserts body "{}", gets "") and passes after — confirming it pins the exact bug.
• Manual end-to-end repro from the issue document: aws lambda invoke without --payload returns {"statusCode":200,"body":"ok"} after the fix instead of the JSON-parse SyntaxError.

Checklist

• ./mvnw test passes locally
• New or updated integration test added
• Commit messages follow Conventional Commits

[hectorvent]

Great catch @rhrlima,

Thanks