Add Skip Null Handling when ToString

[marcellinuselbert]

This PR is for the issue #3908

This changes accommodate the users to skip print the model's null value field

note: can you guys give me insights on how to make a unit test or how to test the annotation? thank you!

[Rawi01]

To create a test you have to add source file to test/transform/resource/before (e.g. ToStringSkipNull.java) and a corresponding file to test/transform/resource/after-(delombok|ecj). The test system transforms the before file and compares the output with the after file.

While I appreciate your motiviation to support lombok, please search open and closed issues and PRs before starting to work on something. There are already multiple issues (e.g. #1297, #1752) to extend @ToString and most of them do not pass the requirements to become a lombok feature.