api/push: Don't barf on non-existent jobsets

Dependencies

• [2] PCKLFRT5 Support push notification of repository changes
• [3] 4SFOPTHW Use query_params instead of params
• [*] BKOIYITR added some json responses

Change contents

• replacement in src/lib/Hydra/Controller/API.pm at line 307
  [2.773]→[2.773:955](∅→∅)
  − my $jobset = $c->model('DB::Jobsets')->find($p, $j) or notFound($c, "Jobset ‘$p:$j’ does not exist.");

  − next unless $jobset->project->enabled && $jobset->enabled;
  [2.773]

  [2.955]
  + my $jobset = $c->model('DB::Jobsets')->find($p, $j);

  + next unless defined $jobset && $jobset->project->enabled && $jobset->enabled;