always include a walk/vehicleRentalState=null state in initial states

[tkalvas]

Summary

Alternative fix for #6054 .

Generate an initial state with currentMode = WALK, vehicleRentalState = null in addition to the others, when requestMode.includesRenting() and arriveBy = true.

I think this solution changes some general assumptions in what states are possible, and should perhaps be discarded purely on that basis. Earlier, I think it was the case that if requestMode.includesRenting(), vehicleRentalState is never null, it's always either BEFORE_RENTING, HAVE_RENTED, RENTING_FROM_STATION, or RENTING_FLOATING, and the expectation was that it is allowed for BEFORE_RENTING or HAVE_RENTED to reach the goal without ever actually have been renting anything, and it's still a success. Now, it can be null, and it is possible that for correctness every state.getRequest().mode().includesRenting() should be state.getRequest().mode().includesRenting() && state.getVehicleRentalState() != null instead. I only changed the one at the beginning of StreetEdge.traverse, and it seems to work, but it feels incomplete.

Issue

Closes #6054

Unit tests

Because of the above mentioned expectation on the link between includesRenting and vehicleRentalState, some unit test changes were forced.

Documentation

None.

Changelog

Yes, fixes a real bug.

Bumping the serialization version id

No.

[tkalvas]

An alternative fix is at #6233 and I prefer it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.72%. Comparing base (5e7b3a6) to head (987d204).
Report is 163 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6244      +/-   ##
=============================================
- Coverage      69.74%   69.72%   -0.02%     
- Complexity     17646    17665      +19     
=============================================
  Files           2006     2007       +1     
  Lines          75529    75650     +121     
  Branches        7730     7743      +13     
=============================================
+ Hits           52678    52748      +70     
- Misses         20135    20187      +52     
+ Partials        2716     2715       -1     

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