SitemapSpider throws lxml.etree.XMLSyntaxError when trying to scrape a blank sitemap page

[Endi1]

Description

SitemapSpider throws a lxml.etree.XMLSyntaxError when hitting a blank sitemap page while crawling a sitemap.

Example sitemap with blank pages: https://bikeradar.com/sitemap.xml

Stack trace

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/scrapy/utils/defer.py", line 102, in iter_errback
    yield next(it)
  File "/usr/local/lib/python3.8/site-packages/scrapy/core/spidermw.py", line 84, in evaluate_iterable
    for r in iterable:
  File "/usr/local/lib/python3.8/site-packages/sh_scrapy/middlewares.py", line 30, in process_spider_output
    for x in result:
  File "/usr/local/lib/python3.8/site-packages/scrapy/core/spidermw.py", line 84, in evaluate_iterable
    for r in iterable:
  File "/usr/local/lib/python3.8/site-packages/scrapy/spidermiddlewares/offsite.py", line 29, in process_spider_output
    for x in result:
  File "/usr/local/lib/python3.8/site-packages/scrapy/core/spidermw.py", line 84, in evaluate_iterable
    for r in iterable:
  File "/usr/local/lib/python3.8/site-packages/scrapy/spidermiddlewares/referer.py", line 339, in <genexpr>
    return (_set_referer(r) for r in result or ())
  File "/usr/local/lib/python3.8/site-packages/scrapy/core/spidermw.py", line 84, in evaluate_iterable
    for r in iterable:
  File "/usr/local/lib/python3.8/site-packages/scrapy/spidermiddlewares/urllength.py", line 37, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/usr/local/lib/python3.8/site-packages/scrapy/core/spidermw.py", line 84, in evaluate_iterable
    for r in iterable:
  File "/usr/local/lib/python3.8/site-packages/scrapy/spidermiddlewares/depth.py", line 58, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/usr/local/lib/python3.8/site-packages/scrapy/spiders/sitemap.py", line 53, in _parse_sitemap
    s = Sitemap(body)
  File "/usr/local/lib/python3.8/site-packages/scrapy/utils/sitemap.py", line 18, in __init__
    self._root = lxml.etree.fromstring(xmltext, parser=xmlp)
  File "src/lxml/etree.pyx", line 3234, in lxml.etree.fromstring
  File "src/lxml/parser.pxi", line 1876, in lxml.etree._parseMemoryDocument
  File "src/lxml/parser.pxi", line 1764, in lxml.etree._parseDoc
  File "src/lxml/parser.pxi", line 1127, in lxml.etree._BaseParser._parseDoc
  File "src/lxml/parser.pxi", line 601, in lxml.etree._ParserContext._handleParseResultDoc
  File "src/lxml/parser.pxi", line 711, in lxml.etree._handleParseResult
  File "src/lxml/parser.pxi", line 640, in lxml.etree._raiseParseError
  File "<string>", line 1
lxml.etree.XMLSyntaxError: Document is empty, line 1, column 1

Steps to Reproduce

1. Crawl sitemap with at least a blank .xml page

Expected behavior:
Handle the XMLSyntaxError by logging a warning for an invalid sitemap page and returning.

Actual behavior:
Throws XMLSyntaxError

Versions

Scrapy version 1.7.2

[Yogita98]

Is this issue up for grabs?

[Gallaecio]

There has been an attempt at it, #4214, but it seems stale. Feel free to give it a try, either from scratch or based on #4214.