Ktor Jackson: when sending empty content and using receive with type an exception occurs

I found an error in ktor feature using jackson. It does not handle empty body content very well.

I have this example application:

```
data class Han(val x: Int)

fun main(args: Array) {
val server = embeddedServer(Netty, port = 8080) {
install(ContentNegotiation) {
jackson {
propertyNamingStrategy = PropertyNamingStrategy.LOWER_CAMEL_CASE
}
}

routing {
post("/demo") {
val t = call.receive(Han::class)
}
}
}
server.start(wait = true)
}
```

When i send with postman an empty body with contentType: `application/json` to `/demo` it crashes with the following exception:

```
com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input
at [Source: (InputStreamReader); line: 1, column: 0]
at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
at com.fasterxml.jackson.databind.ObjectMapper._initForReading(ObjectMapper.java:4133)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3988)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3037)
at io.ktor.jackson.JacksonConverter.convertForReceive(JacksonConverter.kt:40)
at io.ktor.features.ContentNegotiation$Feature$install$3.doResume(ContentNegotiation.kt:95)
at io.ktor.features.ContentNegotiation$Feature$install$3.invoke(ContentNegotiation.kt)
at io.ktor.features.ContentNegotiation$Feature$instal...
```

I think this should be handled more gracefully than this hard exception.

Thank you guys for all your effort! Ktor is awesome.

Versions:
```
implementation("io.ktor:ktor-server-netty:0.9.5")
implementation("io.ktor:ktor-server-core:0.9.5")
implementation("io.ktor:ktor-jackson:0.9.5")
```
with kotlin `1.2.71`

Start with io/ktor/jackson/JacksonConverter.kt at convertForReceive and reproduce the empty application/json request from the example against POST /demo. Check how ContentNegotiation.kt handles the converter result and define a test for the empty-body case; done means it is handled gracefully rather than producing the shown Jackson exception.

Written by the indexing model from the issue text.