LRS should return `200 OK` to the GET statements when no statement present

[wilbrdt]

Bug Report

Expected behavior/code
As specified here, when the LRS contains no statements, the LRS should still return 200 OK to a GET Statements (with an empty array of statements).

Actual Behavior
LRS returns a 500 Internal Server Error.

Steps to Reproduce

bin/ralph runserver -b es
http -a janedoe:supersecret :8100/xAPI/statements

Environment

• Ralph version: 3.5.0
• Platform: -