[c++][fix] check nullable of bin mappers in dataset_loader.cpp (fix #5221) (#5258)
[c++][fix] check nullable of bin mappers in dataset_loader.cpp (fix #5221) (#5258)

check nullable of bin mappers
henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM