Preconditions

1. In a less file if you use &:not(:first-child) it generates spacing warnings.

Steps to reproduce

1. Use &:not(:first-child) in a less file.
2. Run the coding standard from command line. vendor/bin/phpcs --standard=Magento2 path/to/module

Expected result

1. No Warnings are generated.

Actual result

1. Warnings are generated
   154 | WARNING | Expected 1 space after colon in style definition; newline found
   154 | WARNING | Expected 1 space after colon in style definition; 0 found