Relevant language on DC delegate selection per TheGreenPapers.com.
“If a Presidential candidate who is allocated delegates withdraws prior to the National Convention, the delegates bound to that Candidate become unbound. [Section V. 7.]. If only one candidate’s name is placed in nomination ... all delegates shall be bound to vote for such candidate on the first ballot provided that the candidate received votes in the DC presidential preference poll [Section V. 8.]. The DC Delegation ... shall be bound through the first ballot .... [Section V. 9.]”
Since Trump is the only nominee, he gets all the unbound delegates.
Thank you, smith!