fix wrong unchecked null cast (potential NPE)

[y9maly]

Problem Description

I encountered an issue where an RPC service returned null even though the RPC function never actually returns null

After investigation, it turned out that the error occurred during the deserialization of the server's response. However, kotlinx-rpc silently ignored the deserialization exception and performed an unchecked cast of null to the RPC function's return type T

This happens inside the KrpcClient class in the handleServerStreamingMessage function:

is KrpcCallMessage.CallSuccess, is KrpcCallMessage.StreamMessage -> {
    val value = runCatching {
        val serializerResult = serialFormat.serializersModule
            .buildContextual(callable.returnType)
        decodeMessageData(serialFormat, serializerResult, message)
    }
    @Suppress("UNCHECKED_CAST")
    channel.send(value.getOrNull() as T) // Thats not ok
}

When deserialization fails, getOrNull() returns null, which is then unchecked-cast to T. This may propagate unexpected null values

Expected behaviour

If deserialization fails, the error should be propagated back to the remote function call site, instead of sending a null value through the channel

Replacing getOrNull() with getOrThrow() would throw the exception immediately inside the library's internals, but this would not deliver it to the location where the developer actually invokes the remote function.
Therefore, I switched from Channel to Channel<Result>, allowing the deserialization error to be captured and then rethrown at the correct call site

This is my first attempt at contributing, please don’t hit me 🙃

[Mr3zee]

Hey @y9maly! Thanks for you contribution! I'll take a look at it tomorrow

[Mr3zee]

@y9maly lgtm! Please write a test, and then I'll run the CI checks. If everything is fine - we'll merge it.

[y9maly]

@Mr3zee i did it

[Mr3zee]

@y9maly thanks! One small change, please, while I'll run the tests

[Mr3zee]

Our CI is broken, so I ran tests locally, and everything seams good.
Thank you for your great contribution!

[Mr3zee]

Will be released in 0.10.0 in upcoming weeks