follow_all fails with an empty list of URLs

[ivanprado]

Description

follow_all with an empty list of urls fails with ValueError('Please supply exactly one of the following arguments: urls, css, xpath')

What I would expect instead is just an empty generator of requests.

Steps to Reproduce

class Spider(scrapy.Spider):

    def parse(self, response):
        yield from response.follow_all([], self.parse)

Expected behavior:

No error is raised

Actual behavior:

ValueError('Please supply exactly one of the following arguments: urls, css, xpath') exception is raised.

Versions

2.0

Additional context

I think the solution is just a matter of changing this line: https://github.com/scrapy/scrapy/blob/master/scrapy/http/response/text.py#L191