fix(avante-nvim): Check for nil value in opts table for markview.nvim dependency

[lucazz]

📑 Description

markview.nvim Plugin Enhancements (Avante integration)

• Added a defensive check for opts.preview to avoid potential nil dereferencing.
• Ensures compatibility and resilience when plugin options are partially defined.

ℹ Additional Information

• This update to markview.nvim ensures that the plugin doesn't fail if opts.preview is undefined, which may occur depending on other plugin configurations.

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).

[Uzaaft]

Brother in christ that PR message and commit message is so bad.

[Uzaaft]

@lucazz Advice: Just keep it simple.

just added null checks

Would have been enough with:

fix(avante-nvim): Check for nil value in dependency opts

as the PR title or commit message or something.