serix: fix empty slice from TxEssenceOutputs JSONEncode

[jkrvivian]

The Encode() works fine on TxEssenceOutputs, but JSONEncode returns an empty slice.

This bug is discovered in #676 .
The outputs assertion in AssertTransaction in test suite reports a difference in a slice of TxEssenceOutputs with 1 account output and 1 foundry output, changing to EqualValues or ElementsMatch all report an error. However, the actual values are the same.
In #676 , outputs are now checked after Encode()

iota-core/pkg/testsuite/transactions.go

Line 41 in 6d2857a

if !assert.Equal(t.fakeTesting, transaction.Outputs, typedTransaction.Outputs) {

[jkrvivian]

JSON doesn't support the encoding of arrays, only objects, thus an empty slice is returned.