crash on reference like structure before directive with parenthesis

[ChristianMurphy]

Subject of the issue

const micromark = require("micromark/lib");
const directive = require("micromark-extension-directive");

micromark( "[!]:)", "utf-8", { extensions: [directive()] });

throws

AssertionError [ERR_ASSERTION]: expected non-empty token (`chunkString`)

Your environment

• OS: Ubuntu
• Packages:

  ├── micromark-extension-directive@1.1.0
  └── micromark@2.11.2

• Env: node v15.2.1, npm 7.0.8

Steps to reproduce

run:

const micromark = require("micromark/lib");
const directive = require("micromark-extension-directive");

micromark( "[!]:)", "utf-8", { extensions: [directive()] });

Expected behavior

No error, or a more specific markdown syntax related error

Actual behavior

AssertionError [ERR_ASSERTION]: expected non-empty token (`chunkString`)

[wooorm]

This is a bug in mm apparently! definitions!

[wooorm]

A clearer example is [x]: )

[wooorm]

Fixed! Thanks for the wonderful crash reports @ChristianMurphy!