Among the crop of summer Off-Broadway musicals, and it’s been a flavorless crop, here’s something of an anomaly. The Sabbath Girl (book by Cary Gitter, lyrics by Gitter and Neil Berg, music by Berg) isn’t overproduced like Empire, or bathetic like From Home. Whatever its deficiencies, and it does have them, The Sabbath Girl also has something we haven’t been seeing in a lot of new musicals: it has a heart.