html cleaner assert parent is not None AssertionError

[guyskk]

The html cleaner raise AssertionError when both root tag and child tag in kill_tags set.

Reproduce:
```
from lxml.html.clean import Cleaner

html_cleaner = Cleaner(kill_tags=['pre', 'code'])

content = '<pre><code>xxx</code></pre>'
print(html_cleaner.clean_html(content))
```

Output:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "src/lxml/html/clean.py", line 520, in lxml.html.clean.Cleaner.clean_html
  File "src/lxml/html/clean.py", line 394, in lxml.html.clean.Cleaner.__call__
  File "/Users/kk/.pyenv/versions/rssant/lib/python3.7/site-packages/lxml/html/__init__.py", line 339, in drop_tree
    assert parent is not None
AssertionError
```

Expected output:
```
<div></div>
```

I tested in Python 3.7.4, lxml-4.3.3 and lxml-4.4.1.

Python : sys.version_info(major=3, minor=7, micro=4, releaselevel='final', serial=0)
lxml.etree : (4, 4, 1, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)

The bug seems similar to https://bugs.launchpad.net/lxml/+bug/773715, but occurs in different line.

[scoder (scoder)]

Hmm, yes, it's probably worth reconsidering that assertion. In the end, what you're asking for is to discard the entire "document". That's a valid thing to do, and it should not run into an AssertionError.
OTOH, what should it return in such a case?
Maybe it could be enough to remove all content from the tag (children and inner text), and then still return it?

[guyskk (guyskk)]

It's OK to return empty div in the case.

When I remove the code tag in the input content, it return empty div, I think it's fine.
lxml only raise AssertError when both root tag and child tag in kill_tags set.

```
>>> from lxml.html.clean import Cleaner
>>> html_cleaner = Cleaner(kill_tags=['pre', 'code'])
>>> content = '<pre>xxx</pre>'
>>> print(html_cleaner.clean_html(content))
<div></div>
```