Correctly omit transactionData field when transaction simulation fails

[Shaptic]

What

Correctly handle the case where transaction data is empty. Also, add a bit of verbosity (the type name) to error messages coming from the conversion layer.

Why

Previously, the handler would unconditionally try to encode an empty byte array (since result.TransactionData was empty) and fail with an internal panic:

"xdr_to_json() failed: called `Result::unwrap()` on an `Err` value: Io(Error { kind: UnexpectedEof, message: \"failed to fill whole buffer\" })",

Obviously this is not ideal. It was occurring because the simulation failed, but the code would still try to JSONify results. In the base64 case, EncodeToString will just return an empty string on an empty buffer.

This code modifies xdr2json.ConvertBytes to check its inputs like so:

 func ConvertBytes(xdr interface{}, field []byte) ([]byte, error) {
+       if len(field) == 0 {
+               return []byte(""), nil
+       }
+

to behave akin to EncodeToString and handle this case "in general."

[sreuland]

Alternatively, we could modify xdr2json.ConvertBytes to check its inputs

that approach of sanitizing the json lower in conversion to empty string seems to be a bit cleaner, results in less conditionals in upper code, I would 👍 for that.

[Shaptic]

@sreuland agreed! Done ✔️

[psheth9]

can you add few tests? overall looks good to me just one question on rust code

[sreuland]

lgtm, for consideration, echo the same as other reviewer commented, about adding unit test on conversion.go if feasible.

[Shaptic]

Good calls @sreuland and @psheth9! Done :)