Support nullable contextual serializers

[yakivy]

Context: https://kotlinlang.slack.com/archives/C072YJ3Q91V/p1752238186483329

[Mr3zee]

@yakivy Hey! Thanks for the contribution! I'll accept that, but can you please move the test into krpc/krpc-test/src/commonMain/kotlin/kotlinx/rpc/krpc/test/KrpcTransportTestBase.kt? Sampling is for the protocol testing purposes

You can see how LocalDateSerializer is used there for contextual serialization. Tests don't run in IDE, so you can check it with ./gradlew krpc:krpc-test:jvmTest --tests "*<test_name>*"

[yakivy]

@Mr3zee lol, I knew there should be an easier way! Thanks for pointing out. Could you take a look?

[Mr3zee]

@yakivy yep, thanks! One last thing: please put the test in the base class and just ignore it in protobuf. You can see how it's done for other nullable test

[yakivy]

@Mr3zee done

[Mr3zee]

@yakivy thanks for the contribution!