fix: allow nil values on grid cover

[Paul-Bob]

Description

Fixes #1816

When cover type is file check if value is present before calling attached? on it. Before this PR having a nil value on grid cover would break the app, now it renders the Avo::Index::GridCoverEmptyStateComponent

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Manual review steps

1. Step 1
2. Step 2

Manual reviewer: please leave a comment with output from the test if that's the case.

[codeclimate]

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[adrianthedev]

can we ensure that this is happening on file and files field too please?

[Paul-Bob]

can we ensure that this is happening on file and files field too please?

Both file and files was accepting nil on all pages. The issue was only on grid do block the cover was breaking if nil value present. We don't accept cover :id, as: :files although can be an interesting feature to have an carousel of images on the grid cover configuration.

[github-actions]

This PR has been merged into main. The functionality will be available in the next release.

Please check the release guide for more information.