Fix nullability problem wrt return type of kotlin services

[slinkydeveloper]

Fix #316

[github-actions]

Test Results

 48 files  +34   48 suites  +34   2m 55s ⏱️ + 2m 45s
738 tests +76  731 ✅ +76  7 💤 ±0  0 ❌ ±0 
738 runs  +75  731 ✅ +75  7 💤 ±0  0 ❌ ±0 

Results for commit c6960b5. ± Comparison against base commit efba4ec.

This pull request removes 7 and adds 83 tests. Note that renamed tests count towards both.

dev.restate.sdk.TestSerdesTest ‑ Double: 0.3756986588909651
dev.restate.sdk.TestSerdesTest ‑ Float: 0.55834234
dev.restate.sdk.TestSerdesTest ‑ Integer: 79630332
dev.restate.sdk.TestSerdesTest ‑ Long: 4257690258002159025
dev.restate.sdk.TestSerdesTest ‑ Short: 9792
dev.restate.sdk.http.vertx.HttpVertxTests ‑ 225: [HttpVertxTestExecutor][dev.restate.sdk.http.vertx.VertxExecutorsTest] CheckNonBlockingComponentTrampolineExecutor#do
dev.restate.sdk.http.vertx.HttpVertxTests ‑ 226: [HttpVertxTestExecutor][dev.restate.sdk.http.vertx.VertxExecutorsTest] CheckBlockingComponentTrampolineExecutor#do

dev.restate.e2e.JavaAwaitTimeoutTest ‑ Test Awaitable#await(Duration)
dev.restate.e2e.JavaAwakeableTest ‑ generate(IngressClient)
dev.restate.e2e.JavaCancelInvocationTest ‑ cancel blocked invocation on AWAKEABLE
dev.restate.e2e.JavaCancelInvocationTest ‑ cancel blocked invocation on CALL
dev.restate.e2e.JavaCancelInvocationTest ‑ cancel blocked invocation on SLEEP
dev.restate.e2e.JavaCoordinatorWithNodeReceiverServiceToServiceCallTest ‑ synchronousCall(IngressClient)
dev.restate.e2e.JavaErrorsTest ‑ Test calling method that fails terminally
dev.restate.e2e.JavaErrorsTest ‑ Test calling method that fails terminally multiple times
dev.restate.e2e.JavaErrorsTest ‑ Test invocations are retried until they succeed
dev.restate.e2e.JavaErrorsTest ‑ Test propagate failure from another service
…

♻️ This comment has been updated with latest results.