[http-client-java] In toJson, we should skip writeUntypedField if value is null for optional properties

[XiaofeiCao]

Describe the bug

Azure/azure-sdk-for-java#44686 (comment)

This should happen for both branded/unbranded, dpg and mpg.

Reproduction

See above.

Checklist

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• The provided reproduction is a minimal reproducible example of the bug.

[weidongxu-microsoft]

We may first check with core whether writeUntypedField would skip null in its internal impl, as other types like writeStringField would skip the field, if value is null.

[XiaofeiCao]

It'll not skip and seems by design:
https://github.com/Azure/azure-sdk-for-java/blob/282a1cc1910eb40a79e81aa0ce5f3fbd064203d5/sdk/serialization/azure-json/src/main/java/com/azure/json/JsonWriter.java#L1060