For someone who has been acting in plays since he “was knee-high to a grasshopper,” the 91 years-young John Cullum has lost none of his love for performing. That is evident in “John Cullum: An Accidental Star.” Conceived by Cullum and Jeff Berger and written by David Thompson, the show is a vehicle for the Tennessee-born Cullum to look back at some of what he calls the lucky accidents which befell him in his career. Offering these recollections with a slightly self-deprecating delivery, these tales both gently envelop and engage the viewer. After a brief musical intro, Cullum, who arrived in New York in 1956, relates how he got his first theater job in the Big Apple as a spear carrier in an off-Broadway production of “Saint Joan.” His second job offer came soon after in a production of “Hamlet.” Cullum still has the book of Shakespeare plays he bought …