anchors_plugin: a heading with no slug-able characters gets an empty id="" attribute #175

What's wrong

anchors_plugin generates an empty id attribute for any selected-level heading whose title has no slug-able characters, e.g. ## ???:

from markdown_it import MarkdownIt
from mdit_py_plugins.anchors import anchors_plugin

md = MarkdownIt().use(anchors_plugin)
md.render("## ???\n")
# '<h2 id="">???</h2>\n'

An empty id is invalid HTML (the value must not be the empty string), and a second such heading gets the odd id="-1" (the empty string deduplicated):

md.render("## ???\n\n## ???\n")
# '<h2 id="">???</h2>\n<h2 id="-1">???</h2>\n'

With permalink=True it also emits a link that points nowhere:

md = MarkdownIt().use(anchors_plugin, permalink=True)
md.render("## ???\n")
# '<h2 id="">??? <a class="header-anchor" href="#">¶</a></h2>\n'

The plugin already documents the right doctrine for the explicit-ID path — "an empty explicit ID counts as no ID" (test_explicit_heading_id_edge_cases even asserts 'id=""' not in rendered and 'href="#"' not in rendered) — but the generated-slug path never checks for an empty result, so the same invalid HTML slips in from slugify() (a title of only non-word characters like ???, ***, or $$$ slugifies to "").

Expected

A heading whose generated slug is empty should be treated the same way as an empty explicit ID: no id attribute at all, and no permalink for it (href="#" would point at the top of the page).

Environment: mdit-py-plugins 0.7.0 (master fde61f4), markdown-it-py 3.0.0.