LLCSSBG image isnot applied on relative CSS file of subsite in multisite

[Mai-Saad]

Describe the bug
Lazyload bg image optimization is not applied to relative CSS file containing bg image on the subsite of multisite

To Reproduce
Steps to reproduce the behavior:
1- Have page template with relative CSS file that has bg images i.e <link rel="stylesheet" href="/wp-content/rocket-test-data/styles/style-single-colon.css">
2- enable llbgcss
3- visit the URL in a subsite of a multisite subdirectory setup i.e https://newmulti.rocketlabsqa.ovh/test1/page
4- check source => URL is not rewritten to llbgcss folder

Expected behavior
LLBGCSS is applied to relative CSS containing bg image in a subsite of multisite

Additional context

• visit the same page on the main site => the css file will be rewritten to llbgcss folder
• if the file URL is an absolute URL, the file will be rewritten to the CSS bg folder
• todo: check on multisite subdomain