Referrer URL is empty

This is actually not an issue. There might be several reasons why the referer URL would be blank. It will/may be empty when the enduser:

entered the site URL in browser address bar itself.
visited the site by a browser-maintained bookmark.
visited the site as first page in the window/tab.
switched from a https URL to a http URL.
switched from a https URL to a different https URL. (only if it is blocked by referrer metatag on website)
has security software installed (antivirus/firewall/etc) which strips the referrer from all requests.
is behind a proxy which strips the referrer from all requests.
visited the site programmatically (like, curl) without setting the referrer header (searchbots!).