fix null ptr exception in feed/get/{id} rest call when all entries are read
- map of the unread counts does not contain any element for the feed if
  no entry is unread, leading to a null pointer exception when building
  the response
- problem occurs for example when showing the feed details and all entries
  are read: all fields are blank
Merge pull request #411 from MageFroh/null-ptr-fix

Fix null ptr exception in feed/get/{id} rest call when all entries are read