Show proper duration for failed trainings

[omranlm]

Describe the bug
Show empty duration or dash - when there is not finished_at date for a training record to avoid those non sense duration as in the screenshot below

To Reproduce
Steps to reproduce the behavior:

1. Go to a failed training that doesn't have a finished_at value
2. see the duration calculation which is calculating wrong value

Expected behavior
Show a "-" or empty value when training is failed and there is not finished_at value

Screenshots

[omranlm]

@jeafreezy

[jeafreezy]

PR up for this @omranlm . #298

[jeafreezy]

@jeafreezy

Great catch! Thanks.