Update Style/NilLambda to handle procs as well

[dvandersluis]

Similarly to #9775, updates Style/NilLambda to handle procs as well.

Like I wrote in #9775 (comment), the naming is a bit off here, but I think it's better than the two alternatives -- create a separate Style/NilProc cop (duplication) or rename Style/NilLambda to NilLambdaAndProc or something (breaking change).

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.