In summers past, Warner Brothers used Comic-Con International , the premiere convention for comic book, science fiction and fantasy fans, as a marketing platform for movies like “ Sherlock Holmes,” “300” and “Sucker Punch.” Enlarge This Image Walt Disney Studios staged “ Tron: Legacy ” stunts there three years in a row. Last July, DreamWorks Animation paraded Will Ferrell, Tina Fey and other members of the “ Megamind ” cast through the convention. This year? Warner’s main studio operation is bringing nothing. Ditto Disney and DreamWorks. The Weinstein Company, a perennial presence, will also sit this one out. Even Marvel Entertainment, whose panel for “ The Avengers ” was a highlight of Comic-Con 2010, is on the fence about whether it will mount a major presentation. Comic-Con, as a growing number of movie marketers are realizing, has turned into a treacherous place. Studios come seeking buzz, but the Comic-Con effect can be more negative than positive. The swarm of dedicated f