fix(html2text): keep the word boundary an empty emphasis pair swallowed

[L4XB]

Summary

A whitespace-only inline element leaves an empty marker pair in the markdown, and between two
styled runs it takes the only space separating two words with it.

handle_data strips whitespace-only content inside a stressed tag:

if self.stressed:
    data = data.strip()        # "  " -> ""

so <b> </b> reaches the closing tag with the opening ** as the last thing written and
nothing after it. The closing marker is emitted regardless.

Measured with CustomHTML2Text().handle(...):

input	before	after
<p>Hello<b> </b>world</p>	Hello**** world	Hello world
<p>Hello<em> </em>world</p>	Hello __ world	Hello world
<p>Hello<s> </s>world</p>	Hello~~~~ world	Hello world
<p><b>First</b><b> </b><b>Last</b></p>	**First********Last**	**First** **Last**
<p><em>First</em><em> </em><em>Last</em></p>	_First_ ___Last_	_First_ _Last_

The last two are the ones that matter: an editor that emits one element per styled run puts
the space between two bold words in an element of its own, and both rows above lose it
entirely. **First********Last** renders as FirstLast, and ___Last_ opens bold-italic
that never closes.

List of files changed and why

• crawl4ai/html2text/__init__.py — new drop_empty_stress(), called from the three closing
  branches (em/i/u, strong/b, del/strike/s). It pops the opening marker back off
  outtextlist when that marker is still the last thing written, and restores the whitespace
  the marker stood for. Taking an already-emitted token back out of outtextlist is the
  technique the anchor handler in this file already uses for an empty link.
• tests/unit/test_html2text_empty_emphasis.py — new.

The guard only fires when the opening marker is still the last element, so an element with
real content never reaches it. <b>a</b><b>b</b> also stays **a****b**: there is no
whitespace between those runs in the HTML, so the words really are adjacent.

How Has This Been Tested?

tests/unit/test_html2text_empty_emphasis.py, 19 cases:

• the whitespace-only element across all eight tags (b strong em i u s del strike)
• the space between two styled runs, for five of them, asserting the exact markdown
• whitespace already present outside the element is not doubled, on both sides
• five control cases with real content, including the nested <b><i>x</i></b> and the
  genuinely adjacent <b>a</b><b>b</b>

14 of the 19 fail on main; the 5 that pass are the controls. Seven mutants of the changed
lines are killed by them, one per call site plus the whitespace restore in both directions,
the " " + mark match, and the preceding_stressed reset.

tests/unit/test_html2text_empty_emphasis.py          19 passed
tests/test_markdown_generator_validation_1880.py
  + tests/test_table_gfm_compliance.py               24 passed
tests/unit                                           same 28 pre-existing failures as main
                                                     (pdf and sitemap optional deps)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes