Fix update method in post service

[camelya58]

Please, fix code in update method of PostServiceRealization class.
When you try in swagger to update some post, for example, with id "5f390c7e810fe1a026e7f80", which doesn't exist in repository, you receive incorrect response.
Request:

curl -X PUT "http://post-reckue.apps.us-east-1.starter.openshift-online.com/posts/5f390c7e810fe1a026e7f80" -H "accept: */*" -H "Content-Type: application/json" -d "{ \"nodes\": [ { \"node\": { \"type\": \"TEXT\" }, \"postId\": \"string\", \"source\": \"string\", \"type\": \"TEXT\", \"userId\": \"string\" } ], \"source\": \"string\", \"status\": \"DRAFT\", \"tags\": [ { \"id\": \"string\", \"name\": \"string\" } ], \"title\": \"string\", \"userId\": \"string\"}"

Expected response:
404 Not Found

{
  "title": "PostNotFoundException",
  "message": "Post by id '5f390c7e810fe1a026e7f80' is not found",
  "code": "RPE-001",
  "trace": "com.reckue.post.exceptions.models.post.PostNotFoundException: Post by id '5f390c7e810fe1a026e7f80' is not found..."
}

Actual response:
505 Internal Server Error

{
  "timestamp": "2020-09-08T20:03:17.714+0000",
  "status": 500,
  "error": "Internal Server Error",
  "message": "No value present",
  "path": "/posts/5f390c7e810fe1a026e7f80"
}

[camelya58]

When you fix it, please describe it in already existed docs.