“ The Devil Wears Prada 2 ” commits a sin that would give any glossy-magazine profile writer a shudder of recognition. It falls in love with its subject — and delivers, rather than a frank and delicious portrait, a puff piece. In this franchise’s first installment, released in 2006, editor-from-hell Miranda Priestly ruled her domain with a terrifying, unflappable calm. That her standing in the publishing world are diminished 20 years later reflects the reality of the print business. But that she reacts to her misfortunes with a practically cuddly amiability indicates that this sequel, determined to super-serve fans what they seem to want, has lost the plot, and the spirit of who Miranda is (or was).  As once again played by Meryl Streep, Miranda enters the sequel recognizably enough.…