fix(recipe): guard against nil instruction_sections in update_metadata

[charliecreates]

Context

Sentry issue RECIPIN-5 (linked from Linear REC-10) raised NoMethodError: undefined method 'each' for nil originating from Recipe#update_metadata when instruction_sections= was called with nil.

Changes

• Updated Recipe#update_metadata to strip out nil values from extractor data and convert a nil instruction_sections value to an empty array before delegating to assign_attributes.
• Added an explanatory inline comment for future maintainers.
• Added unit test to ensure saving a recipe when the extractor returns instruction_sections: nil does not raise and results in an empty association.

Verification

bundle exec rake test

All tests (existing + new) pass locally.

Closes REC-10