[ANCHOR-784] Remove 0 amounts from SEP-6 transaction responses

[philipliu]

Description

This removes the 0 amount_out and 0 amount_fee placeholder values from the SEP-6 exchange transaction responses.

Context

This was set as a placeholder because the request funds RPCs expected these fields always to be set.

Testing

• ./gradlew test

Documentation

N/A

Known limitations

N/A