Regression: crash when rendering JSON binary transaction with API v.2 enabled

[Bronek]

Issue Description

In PR #4775 the API version 2 output of tx method has significantly changed, and in the process has also the JSON type of output from STTx::getJson . If we enable both API v. 2 and binary output, that JSON is no longer an object but a string. It follows that it cannot have any additional JSON fields added, and yet we try to add fields inside Transaction::getJson which causes a crash.

Steps to Reproduce

Run tx method requesting binary output with api_version 2

Expected Result

Output transaction data with tx_blob field (among others)

Actual Result

Crash.