I think historians accept that King Soloman existed, but the criteria for historians to affirm that someone existed is based on the way people wrote about that person. If people wrote about a person as an actual person and there is no other evidence to suggest that the person didn't exist, historians will by default accept them as being real people. Since the Bible says Jesus lived, historians, on the whole, except that he did and the same would be true of Solomon. This doesn't mean we should accept their methodology on this, but I understand why they have to.