Support endless method definitions

[rkh]

Description

Ruby 3.0 introduces endless method definitions:

def method = "return value"

This raises an exception in YARD, as the Ruby parser never calls on_params, thus there is no ParameterNode inside the method definition (see #1376). I've adjusted the code slightly to be able to handle this case.

I've added a test and have also confirmed on a real world project that with this change YARD correctly handles endless method definitions without parameters.

Endless method definitions with parameters were already supported. I've also added a test for them, as there was none.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[coveralls]

Coverage decreased (-0.02%) to 93.404% when pulling 5bbbd9e on rkh:rkh-ruby-3 into 3590066 on lsegal:main.

[rkh]

The coverage report decrease is a false positive (false negative?), as the lines in question are only executed with Ruby 3.0 or higher.

[lsegal]

Thanks for your PR! Sometimes coverage acts up on small deltas.