Handle empty string param in method helper

[jamuc]

Description

Hi, I'm using YARD in combination with yard-cucumber to document a company internal GEM. When I ran YARD I got a nil pointer exception

1) YARD::Templates::Helpers::MethodHelper#format_constant when a empty string is passed as param
     Failure/Error: sp = value.split("\n").last[/^(\s+)/, 1]

     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./lib/yard/templates/helpers/method_helper.rb:70:in `format_constant'
     # ./spec/templates/helpers/method_helper_spec.rb:115:in `block (4 levels) in <top (required)>'
     # ./spec/spec_helper.rb:127:in `block (2 levels) in <top (required)>'

The problem is that somewhere an empty string is being passed to MethodHelper#format.

This PR fixes that issue with the most simple solution I could think of. If Array#last returns nil, default to an empty string.
I've also extended the MethodHelper spec to cover this case.

My current environment

• ruby 3.0.2
• OS X 12.0.1 Monterey

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).

[lsegal]

LGTM!