Signature Theater’s Off Broadway production of Edward Albee’s “Occupant” has sold out even before previews begin in May. Run has been extended a week to July 6.Play, about artist Louise Nevelson, stars Mercedes Ruehl and Larry Bryggman. Pam MacKinnon directs. Quick sellouts have become a common occurrence at Signature, which for the last few years has charged ultra-low prices under the Signature Ticket Initiative, sponsored by Time Warner Inc. Ducats were $20 for the original eight-week engagement of “Occupant;” for perfs during the extension, the pricetag goes up to $65. “Occupant” begins previews May 6 ahead of a June 5 opening.