fix: give a heading with no slug-able characters no anchor at all - #176

Fixes #175

What's wrong

anchors_plugin generates an empty id attribute for any selected-level heading whose title has no slug-able characters (## ???, ## ***, ## $$$ …), which is invalid HTML, and a second such heading gets the odd id="-1":

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

With permalink=True it also emits a permalink pointing at href="#" — the top of the page, not the heading.

The plugin already documents this doctrine for the explicit-ID path ("an empty explicit ID counts as no ID"; test_explicit_heading_id_edge_cases asserts 'id=""' not in rendered), but the generated-slug path never checked for an empty slugify() result.

Fix

Treat a generated empty slug exactly like an empty explicit ID: the heading gets no id attribute and no permalink. "" is also no longer added to the slug set (which is what produced id="-1" for a second such heading).

Tests

New test_empty_generated_slug_gets_no_id (3 cases: no slug-able chars, twice, followed by a real heading):

• all 3 fail on master (fde61f4) with 'id=""' in rendered / id="-1",
• pass with this fix, and the following heading keeps a clean id="real" (no real-1),
• pytest tests/test_anchors.py: 28 passed (25 + 3 new),
• full suite: 637 passed, 3 skipped, 1 xfailed (634 + 3 new), plus the same 16 pre-existing data_regression fixture env errors that occur on a clean checkout (markdown-it-py 3.0.0 here, the repo's declared floor).