Hide code of conduct link when none is set

[sikesbc]

What this PR does:

• Fixes the registration form (new/edit) rendering an empty code of conduct modal and nav link when no code of conduct is set on the organization. Adds the same unless @conference.code_of_conduct.blank? guard that every other callsite already uses.

Related upstream issue: snap-cloud#211

Include screenshots, videos, etc.

• N/A: one-line view fix, no UI additions.

Who authored this PR?
@sikesbc

How should this PR be tested?

• Find a conference whose organization has no code of conduct set.
• Visit the new or edit registration page for that conference.
• Confirm there is no "Code of Conduct" modal or nav link on the page.
• Set a code of conduct on the organization and revisit — confirm the modal and link now appear.
• New specs in spec/controllers/conference_registration_controller_spec.rb cover both cases.

Are there any complications to deploying this?

• No migrations or data changes, just a guard clause in a view partial.