Do not attempt to set job next_time when job is nil (#445)

Having an `at`-rule in sidekiq.yml with a date in the past results in a undefined method `next_time' for nil:NilClass.

lib/sidekiq-scheduler/scheduler.rb

@@ -128,6 +128,8 @@ def load_schedule_job(name, config)
             schedule, options = SidekiqScheduler::RufusUtils.normalize_schedule_options(config_interval_type)
 
             rufus_job = new_job(name, interval_type, config, schedule, options)
+            return unless rufus_job
+
             @scheduled_jobs[name] = rufus_job
             SidekiqScheduler::Utils.update_job_next_time(name, rufus_job.next_time)
 

spec/sidekiq-scheduler/scheduler_spec.rb

@@ -698,18 +698,38 @@
     end
 
     context 'at schedule' do
-      let(:config) { ScheduleFaker.at_schedule(at: Time.now + 60) }
+      let(:config) { ScheduleFaker.at_schedule(at: at) }
 
-      it 'adds the job to rufus scheduler' do
-        expect(instance.rufus_scheduler.jobs.size).to eq(1)
-      end
+      context 'in the future' do
+        let(:at) { Time.now + 60 }
 
-      it 'puts the job inside the scheduled hash' do
-        expect(instance.scheduled_jobs.keys).to eq([job_name])
+        it 'adds the job to rufus scheduler' do
+          expect(instance.rufus_scheduler.jobs.size).to eq(1)
+        end
+
+        it 'puts the job inside the scheduled hash' do
+          expect(instance.scheduled_jobs.keys).to eq([job_name])
+        end
+
+        it 'stores the next time execution correctly' do
+          expect(next_time_execution).to be
+        end
       end
 
-      it 'stores the next time execution correctly' do
-        expect(next_time_execution).to be
+      context 'in the past' do
+        let(:at) { Time.now - 60 }
+
+        it 'does not add the job to rufus scheduler' do
+          expect(instance.rufus_scheduler.jobs).to be_empty
+        end
+
+        it 'does not put the job inside the scheduled hash' do
+          expect(instance.scheduled_jobs.keys).not_to include(job_name)
+        end
+
+        it 'stores the next time execution correctly' do
+          expect(next_time_execution).not_to be
+        end
       end
     end