At the beginning of the year, if someone had told you that one of the most interesting hip-hop releases of 2008 would be a free, 19-track mixtape of songs based around a “Seinfeld” concept by a little-known Washington D.C. rapper, you might’ve asked, “What is the deal with this guy?” Well, he’s Wale, and as odd as it sounds, it works. “Nothing” is a wall-to-wall showcase of Wale’s rhyme skills, which are prodigious, and if you ever watched “Seinfeld,” you’ll grin at the interspersed dialogue clips from the show and the clever references to the program sprinkled throughout. (The song titles are plays on the show’s naming convention — tracks include “The Manipulation” and “The Hype.” Elaine Benes even makes a cameo.) How a 24-year-old dude comes up with an idea like this and then executes it so perfectly, who knows? Thank goodness for reruns, I guess.