Distinguish between optional field and field with null value

JSON allows us to express fields that shouldn't be there and fields that could have a null value. These are different things. In Java with cannot have missing fields, so we use `null` to represent both of these states. We're interoping with an external non-Java party that has a schema like this:
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"type": "object",
"properties": {
"nullProp": { "type": [ "string", "null" ] },
"optionalProp": { "type": "string" }
},
"required": [ "nullProp" ]
}
```
because `optionalProp` is not listed, it can be omitted from the JSON, but if it exists, it must have a non-null value:
* valid: `{ "nullProp": null, "optionalProp": "my value" }`
* valid: `{ "nullProp": null }`
* valid: `{ "nullProp": "null value", "optionalProp": "my value" }`
* invalid: `{ "nullProp": null, "optionalProp": null }` // `optionalProp` cannot be null
* invalid: `{ }` // missing required `nullProp`

we need to send valid JSON that adheres to the schema (manual tests www.jsonschemavalidator.net)

In Java we represent this with:
```java
class Data {
@Nullable String nullProp;
@Nullable String optionalProp;
}
```
but the serialization behavior needs to be different for these two fields. We've looked at all the combinations of `serializeNulls` and `@JsonAdapter`, but couldn't figure out how to omit the optional field when its null, while keeping the null field there if it's null.

Here's a test we've been trying to pass with no luck:
```java
import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;
import com.google.gson.*;

class OptionalTest {

class Data {
// TODO @JsonAdapter(NullAdapter.class)?
String nullProp;
// TODO @JsonAdapter(OptionalAdapter.class)?
String optionalProp;
}

private Gson gson;

@BeforeEach void setUp() {
gson = new GsonBuilder()
// TODO .serializeNulls()?
// TODO .registerTypeAdapterFactory()?
.create();
}

@Test void bothNull() {
Data data = new Data();
data.nullProp = null;
data.optionalProp = null;

assertEquals("{\"nullProp\":null}", gson.toJson(data));
}

@Test void filledContents() {
Data data = new Data();
data.nullProp = "some";
data.optionalProp = "thing";

assertEquals("{\"nullProp\":\"some\",\"optionalProp\":\"thing\"}", gson.toJson(data));
}
}
```