Outside of some obvious exceptions - ie. no flag when you just moved to a new city or a yellow flag when you’ve just started to socially get yourself together or when you’ve just had a falling out for external reasons (ie. if your friends betray you or drop you due to political differences) - not having friends should be considered a red flag.

For one thing from a personal pov, having friends is at least some kind of indicator of social activity as well as a sign that you’re able to interact with others and maintain some kind of a relationship. It also is an indicator that you’re actually willing to commit to others.

From a relationship based pov, people with no friends are usually worse communicators, more likely to develop an unhealthy attachment with you, will potentially circle their whole life around you and will very likely have a ton of insecurities and internal issues that you will be thrown right in the middle of.

So honestly, if you don’t have any friends - maybe it isn’t time for a relationship? maybe you should focus on why you don’t have friends and rectify that before dating?