Return relative_url if site.url is an empty string

[ashmaroli]

Summary

Currently, when site.config["url"] is an empty string, :absolute_url proceeds to prepend the result from :relative_url with the empty string and then normalize the result via Addressable.

Instead, this PR proposes to return the result from :relative_url (which is already normalized via Addressable) in this scenario.

Coverage

The expected result is already covered by a test:

jekyll/test/test_filters.rb

Lines 421 to 428 in f3b4cad

	should "be ok with a blank but present 'url'" do
	page_url = "about/my_favorite_page/"
	filter = make_filter_mock(
	"url" => "",
	"baseurl" => "base"
	)
	assert_equal "/base/#{page_url}", filter.absolute_url(page_url)
	end

[mattr-]

@jekyllbot merge +dev

[DirtyF]

@jekyllbot: merge +dev