fix(send): keep the root path when REMOVE_TRAILING_SLASH is enabled

The trailing-slash regex also matched the leading slash of the root URL, so with REMOVE_TRAILING_SLASH set every home page view was stored with an empty url_path: the home page vanished from the paths/entry/exit reports and could not be selected as a funnel step. Skip the match at the start of the path so "/" and "/#hash" are preserved while "/blog/" and "/blog/#hash" are still trimmed.

Closes #4152