Engine cannot handle HTTP status code 204

[tfruehwirth]

Describe the bug
The SEPA engine runs into an internal server error and reports this to the SEPA client (status code 500) if the SPARQL Endpoint responds with code 204. The reason for this is that the SPARQL11Protocol.java processes the responseEntity which is (rightfully) null for responses with responseCode 204.

To Reproduce
Steps to reproduce the behavior:

1. Start GraphDB as a SPARQL Endpoint
2. Start SEPA Engine
3. Use the SEPA Python Client (SEPY) to send a SPARQL Update inserting data
4. See the output of the client application

Expected behavior
No internal server error should be produced if the SPARQL endpoint responds with HTTP status code 204.

System information(please complete the following information):

• OS: Ubuntu 22.04
• Engine version: v0.14.3
• SparqlEndpoint used: GraphDB 10.6.3

Additional context
Add any other context about the problem here.

I used the following endpoint.jpar config to connect to GraphDB as a SPARQL Endpoint:
{ "host": "localhost", "sparql11protocol": { "protocol": "http", "port": 7200, "query": { "path": "/repositories/my-dataset", "method": "POST", "format": "JSON" }, "update": { "path": "/repositories/my-dataset/statements", "method": "POST", "format": "JSON" } } }