ROCK/POP Elton John: ''Breaking Hearts.'' (Geffen GHS 24031.) - An Elton John album can be a lot like a box of Cracker Jack. There's lots of sweet, chewy popcorn inside - and usually a surprise (perhaps two or three). The title cut is the prize this time - a lilting, mournful melody overmatched by Bernie Taupin's lugubrious lyrics. It's rather a modest surprise, though, and a modest effort on the whole by Elton John. True, there's the usual contingent of clever pop tunes - John's a workhorse when it comes to plowing these out. But these tunes aren't good enough to counterbalance the overall blandness of presentation. Recently, Elton John has settled back into giving his lyrics work to Taupin, and most of the instrumentals - apart from his own keyboards - to his reconstituted early ' 70s band. Perhaps he's become too comfortable. His brilliance in penning vivid, rather eccentric tunes is faded just a bit on ''Breaking Hearts.''