Fix parse reward event index error

[isabella618033]

File "/home/bts/text-prompting/prompting/validators/reward/reward.py", line 33, in parse_reward_events
field_names = [field.name for field in fields(reward_events[0])]
IndexError: list index out of range

[p-ferreira]

It would be nice to have some unit tests around this function, two would be enough:

• one test to validate that the function behaves as expected when given reward events
• one test to validate that the function behaves as expected when given no reward events