Avoid NullPointerException in GTFS GraphQL API

[leonardehrenfried]

Summary

It fixes a NPE in BookingInfoImpl when minimumBookingNotice is null.

Issue

Closes #6273

Unit tests

Added. The reason that I didn't add a new file to GraphQLIntegrationTest is that only flex legs can currently have a booking info, which would require me to add a flex leg and update all assertion files.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.75%. Comparing base (805e8d6) to head (91d6784).
Report is 75 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...planner/ext/restapi/mapping/BookingInfoMapper.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6274      +/-   ##
=============================================
+ Coverage      69.74%   69.75%   +0.01%     
- Complexity     17732    17742      +10     
=============================================
  Files           2010     2010              
  Lines          75923    75925       +2     
  Branches        7776     7776              
=============================================
+ Hits           52951    52962      +11     
+ Misses         20261    20255       -6     
+ Partials        2711     2708       -3     

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[miklcct]

Why do you say that only flex legs can have a booking info? It is recommended as long as a stop has pickup_type or drop_off_type as call agency, and this applies to scheduled service (e.g. coach services) as well.

[leonardehrenfried]

In the GTFS spec you can put them anywhere but OTP only adds them to Flex legs. We can of course change that.