fix(markdown): remove empty links when text is deleted

[EricGrill]

Summary

Fixes #7640 - Hidden link remains when deleting link in markdown widget

When a user deletes all text within a link in the visual markdown editor, the empty link markup was being left behind (visible when switching to raw markdown mode).

Problem

Reproduction:

1. Open page with markdown widget
2. Insert a link inside
3. Delete the linked text in rich text mode
4. Switch to markdown mode - the empty hyperlink [](url) remains

Solution

Added a Slate normalizer in withInlines.js that automatically removes link nodes when they become empty (have no text content).

The normalizer:

• Checks if a node is a link element
• Uses Node.string() to get text content
• Removes the node if text is empty
• Falls through to default normalizer otherwise

Files Changed

• packages/decap-cms-widget-markdown/src/MarkdownControl/plugins/inlines/withInlines.js

Test Plan

• Create a link in the visual markdown editor
• Delete all text within the link
• Verify the link wrapper is also removed
• Switch to raw mode and verify no empty []() markup

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