allow null values for jobs, meaning it should build

Dependencies

• [2] AEKIREIH * Updated hydra_eval_jobs for the new evaluator.
• [*] 4N5APGRG * Start of a helper tool to evaluate job expressions efficiently.
• [*] 4LWGZL33

Change contents

• edit in src/c/hydra-eval-jobs.cc at line 179
  [2.2239]

  [5.1503]
  + }

  +

  + else if (v.type = tNull) {

  + // allow null values, meaning 'do nothing'