Fix NPE in TransModel trip plan query

[vpaturet]

Summary

As detailed in #6948, there is a regression in the way invalid locations are handled in the plan query.
This PR fixes the regression.

• Since Cleanup RouteRequest - make it immutable #6653 it is not possible anymore to build an invalid RouteRequest since validation is performed in RouteRequestBuilder (invalid requests throw RoutingValidationException).
  As a consequences some code branches end up with a null request that triggers an NPE.

• This PR fixes the issue by building an empty TripPlan (without itineraries) in the case of an invalid route request.

Additional refactoring: introducing a builder for PlanResponse

Issue

Closes #6948

Unit tests

Added unit tests.

Documentation

No

Changelog

skip

[codecov]

Codecov Report

❌ Patch coverage is 63.75000% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.20%. Comparing base (3281ab7) to head (66c7078).
⚠️ Report is 3 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...nner/apis/transmodel/TransmodelGraphQLPlanner.java	0.00%	18 Missing ⚠️
...ipplanner/apis/transmodel/model/plan/TripType.java	53.84%	6 Missing ⚠️
...ripplanner/apis/transmodel/model/PlanResponse.java	93.93%	0 Missing and 2 partials ⚠️
...anner/routing/api/request/RouteRequestBuilder.java	0.00%	2 Missing ⚠️
...nner/routing/algorithm/mapping/TripPlanMapper.java	87.50%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6961      +/-   ##
=============================================
+ Coverage      72.18%   72.20%   +0.01%     
- Complexity     19839    19856      +17     
=============================================
  Files           2155     2155              
  Lines          80051    80092      +41     
  Branches        8082     8083       +1     
=============================================
+ Hits           57785    57827      +42     
  Misses         19419    19419              
+ Partials        2847     2846       -1     

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