make component entries nullable of multistage bus xml

.../VectoCommon/BusAuxiliaries/BusHVACSystemConfiguration.cs

VectoCommon/VectoCommon/InputData/DeclarationInputData.cs

...oCore/VectoCore/InputData/FileIO/JSON/JSONSubComponent.cs

.../DataProvider/XMLDeclarationBusAuxiliariesDataProvider.cs

...tion/DataProvider/XMLDeclarationComponentsDataProvider.cs

...aration/DataProvider/XMLDeclarationVehicleDataProvider.cs

...jectAdapter/DeclarationDataAdapterCompletedBusSpecific.cs

...VectoCore/InputData/Reader/Impl/CombinedBusAuxiliaries.cs

VectoCore/VectoCore/Models/Declaration/DeclarationData.cs

VectoCore/VectoCore/Models/Declaration/HVACCoolingPower.cs

VectoCore/VectoCore/Models/Declaration/Mission.cs

.../Integration/CompletedBus/CompletedBusSanityCheckTests.cs

...o_vehicle-stage_input_only_component_nullable_entries.xml

VectoCore/VectoCoreTest/VectoCoreTest.csproj

VectoCore/VectoCoreTest/XML/XMLMultistageBusInputDataTest.cs