input: Avoid painting unset gutter background

[MuNeNiCK]

Related to #2411

Description

The line-number gutter is painted as a separate quad on top of the editor background.

When editor.gutter.background is unset, the current implementation falls back to editor_background(). Since the editor already paints that background, translucent colors are composited twice in the gutter and make it appear darker than the text area.

The fallback can also conflict with a background applied through the editor's Styled implementation because the gutter still paints the theme's editor background separately.

This change paints the gutter background only when editor.gutter.background is explicitly configured. When it is unset, the editor background remains visible without an additional paint layer. Themes that define a dedicated gutter background continue to render it as before.

Screenshots

Before:

After:

How to Test

• Run cargo check -p gpui-component.
• Run cargo run and open the Editor story.
• Use a theme with a translucent editor.background and no editor.gutter.background.
• Verify that the gutter and text area have the same visual background.
• Set editor.gutter.background and verify that the explicit gutter background is still rendered.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (if any) is accurate.
• Passed cargo run for story tests related to the changes.
• [N/A] Tested macOS, Windows and Linux platforms performance (not platform-specific).

[MuNeNiCK]

Thanks for merging the PR. I really appreciate it.