Fix HTML generation for RBS constants without values

[oiahoon]

Description

Fixes #1686.

RBS constant declarations carry a type but no Ruby source value. Render a missing constant value as empty content so the default module template can generate the class page instead of raising NoMethodError.

This includes direct helper coverage, a template-level RBS regression, and the main changelog entry.

Validation

• bundle exec rspec spec/handlers/rbs/constant_handler_spec.rb spec/templates/helpers/method_helper_spec.rb spec/templates/module_spec.rb — 26 examples, 0 failures
• bundle exec rake — 2,786 examples, 0 failures; documentation check passed
• Exact CLI reproduction generated Foo.html with BAR and an empty source-value block
• npx --yes @biomejs/biome format — passed
• npx --yes @biomejs/biome lint — passed with four existing warnings in unchanged JavaScript files
• Ruby syntax checks and git diff --check — passed

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]

A very oddly timed duplicate of #1692.