`ebicsRequest` does not handle null data objects in post callback

[swimand]

Issue:
When a response returns as empty, the data.toString('utf-8') throws an error:

node-ebics-client/lib/Client.js

Line 228 in 4a09c8e

const ebicsResponse = response.version(version)(data.toString('utf-8'), keys);

Exception:

TypeError: Cannot read properties of undefined (reading 'toString')
  at processTicksAndRejections (node:internal/process/task_queues:83:21)
  at emitErrorNT (node:internal/streams/destroy:157:8) |  
  |t TLSSocket.emit (node:events:513:28) |  
  at TLSSocket.socketErrorListener (node:_http_client:494:9) |  
  at ClientRequest.emit (node:events:513:28) |  
  at ClientRequest.onRequestEnd (/app/node_modules/rock-req/index.js:76:16) |  
  at /app/node_modules/ebics-client/lib/Client.js:227:59

Solution:

Add a check for if the data is null and either resolve null or reject with a clear message.