comark/file-tree/empty-tree · Give every ::file-tree at least one entry

A ::file-tree with no fenced block, or with an empty one, has no entries to draw, so it renders nothing. This usually means the tree was written outside the block, or the closing :: came too soon.

Incorrect

::file-tree
```
```
::

lint-comark example.mdexample.md:1:1 comark/file-tree/empty-tree::file-tree 1's fenced block is empty.Write one entry per line.✖ 1 problem

Correct

::file-tree
```
ducks/
  feed.ts
  deploy.sh
```
::

lint-comark example.md✔ No problems