Refactor: ensure campaign cover block does not render unless an image is selected

[JoshuaHungDinh]

Resolves GIVE-2148

Description

The Campaign Cover Image should render blank when an image URL is not set.

Affects

Campaign Cover Block

Visuals

Screen.Recording.2025-02-17.at.3.46.14.PM.mov

Testing Instructions

• Create a campaign
• Add the Campaign Cover block
• If an image has NOT been uploaded -> the frontend should NOT render anything a broken image tag.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

[jonwaldstein]

@JoshuaHungDinh this is not meant to be for the block preview, but for the actual front-end rendering to avoid a broken image src being rendered:

[glaubersilva]

@JoshuaHungDinh Since this task is related to the frontend view, I think we should roll back the placeholder on the block editor.

[glaubersilva]

@JoshuaHungDinh Thanks for the changes. This is ready to go!

[rickalday]

Passed QA tests