Eliminating possible NPE + adding safer try--with-resources version + small code cleanup

[ole4ryb]

What

Methods in mappers return null which potentially can cause NPE, also there is a safer version of try-catch introduced: try-with-resources

Why

When method returns null instead of an empty collection it will propagate nulls into other places in code and can potentially trigger NPE. It is better and safer to return an empty collection. Also, try-with-resources will make sure that the steam will be always closed not matter what.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Internal change (non-breaking change with no effect on the functionality affecting end users)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users
• A corresponding change has been made to the Mapping Documentation repository