Return a null object from `AttributeSet#[]`
[ci skip] Add Tip for change_column_null and change_column_default in guides
Merge pull request #15624 from aditya-kapoor/add-warnings-on-guides

[ci skip] Add Tip for change_column_null and change_column_default in guides
:nail_care: wrap the tip from #15624 at 80 chars [ci skip]
Move `attributes` to the `AttributeSet` object.
Add missing test cases for `attribute_method?`
Use with_parse_json_times helper in tests.
Avoid hardcoded magic number in test teardown.
Merge pull request #15857 from zuhao/refactor_activesupport_encoding_test

Avoid hardcoded magic number in test teardown.
Merge pull request #15856 from zuhao/refactor_activesupport_decoding_test

Use with_parse_json_times helper in tests.
Merge pull request #15850 from sgrif/sg-missing-tests

Add missing test cases for `attribute_method?`
Merge pull request #15849 from sgrif/sg-move-attributes

Move `attributes` to the `AttributeSet` object.
Merge pull request #15839 from sgrif/sg-attr-set-null

Return a null object from `AttributeSet#[]`
Create with_env helper for tests.
Merge pull request #15859 from zuhao/refactor_activesupport_caching_test

Create with_env helper for tests.