JsonAdapter nullSafe parameter is ignored by JsonSerializer/JsonDeserializer type adapters

Hi there,

It looks like gson uses TreeTypeAdapter for JsonSerializer/JsonDeserializer type adapters.
TreeTypeAdapter is always nullSafe, so nullSafe value of JsonAdapter annotation is ignored in this case which is at least confusing.

I fixed this locally by adding nullSafe parameter to the TreeTypeAdapter and would love to submit a PR if it need be. Shall I go ahead?

Thanks!