Empty directive labels are dropped on stringify, merging with following text #15

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Affected package

mdast-util-directive 3.1.0, mdast-util-to-markdown 2.1.2, mdast-util-from-markdown 2.0.3, micromark-extension-directive 3.0.2

Steps to reproduce

import {fromMarkdown} from 'mdast-util-from-markdown'
import {toMarkdown} from 'mdast-util-to-markdown'
import {directive} from 'micromark-extension-directive'
import {directiveFromMarkdown, directiveToMarkdown} from 'mdast-util-directive'

const input = ':a[]a.'
const opts = {extensions: [directive()], mdastExtensions: [directiveFromMarkdown()]}
const tree1 = fromMarkdown(input, opts)

tree₁:

{
  "type": "paragraph",
  "children": [
    { "type": "textDirective", "name": "a", "children": [] },
    { "type": "text", "value": "a." }
  ]
}

stringify:

const text2 = toMarkdown(tree1, {extensions: [directiveToMarkdown()]})
// text2: ':aa.\n'

Re-parse:

tree₂:

{
  "type": "paragraph",
  "children": [
    { "type": "textDirective", "name": "aa", "children": [] },
    { "type": "text", "value": "." }
  ]
}

The directive's name has changed from "a" to "aa" (it consumed the
following a). Rendered HTML differs accordingly:

<!-- tree1 -->
<p><span class="directive-a"></span>a.</p>

<!-- tree2 -->
<p><span class="directive-aa"></span>.</p>

Actual behavior

Empty-label directives merge with adjacent text on re-parse.

Expected behavior

Either always emit [] for directives with empty children, or emit enough
trailing context (a space, a zero-width non-joiner, a reference escape) to
prevent the name from merging with following text. The round-trip must
preserve name.

Runtime

node v24.13.1

Package manager

No response

Operating system

Ubuntu (WSL2)

Build and bundle tools

No response