Zero accumulated distance for last break

[jcoupey]

I spotted a situation where having a final break in an open-ended optimization (i.e. without a vehicle end) can result in a zero duration reported for the break step:

$vroom -g -i final_break_without_end.json | jq '.routes[].steps[] | [.type, .duration, .distance]' 
[
  "start",
  0,
  0
]
[
  "job",
  4401,
  79134
]
[
  "job",
  5107,
  96317
]
[
  "break",
  5107,
  0
]
[
  "end",
  5107,
  96317
]

This is related to the fact that we try to compute added distance pro-rata temporis and accumulated travel times do not increase.

Not directly related to #463 but touches the exact same part of the code.