fix: use default values for optional httpPayload if there is no body

[ghostbuster91]

Related PR in alloy: disneystreaming/alloy#163
This closes #1502

TODO:

• Update alloy dependency once released

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

[ghostbuster91]

This currently fails with the linked version of alloy due to:

[info] [http4s] smithy4s.http4s.SimpleRestJsonComplianceSuite
[error] [http4s] - alloy.test#HttpPayloadWithDefault(client|Request): SimpleRestJsonSomeHttpPayloadWithDefault 5ms
[error] [http4s]   Header Content-Type has value `text/plain` but expected `application/json` (modules/tests/src/smithy4s/tests/ProtocolComplianceSuite.scala:180)
[error] [http4s]
[error] [http4s]     [INFO] 17:34:32 [ProtocolComplianceSuite.scala:170] Pass simple string value as is if payload provided
[error] [http4s]
[error] [http4s] - alloy.test#HttpPayloadWithDefault(client|Request): SimpleRestJsonNoneHttpPayloadWithDefault 5ms
[error] [http4s]   Header Content-Type has value `text/plain` but expected `application/json` (modules/tests/src/smithy4s/tests/ProtocolComplianceSuite.scala:180)
[error] [http4s]
[error] [http4s]     [INFO] 17:34:32 [ProtocolComplianceSuite.scala:170] Use default value when there is no payload
[error] [http4s]

[kubukoz]

Have you checked this with the latest changes on the alloy side (content type related)?

[ghostbuster91]

Have you checked this with the latest changes on the alloy side (content type related)?

Yes, twice.

[kubukoz]

Scripted test failing due to a mismatch of the Alloy version btw