Fix accidental inclusion of nil 'at' attribute, fixes #4321

Also, change atomic_push to check for the 'at' key rather than the value of 'at' so this error is caught early and doesn't lead to malformed job payloads in Redis (and tests will fail).

Changes.md

@@ -2,6 +2,12 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
+HEAD
+---------
+
+- Fix `Sidekiq::Client.push_bulk` API which was erroneously putting
+  invalid `at` values in the job payloads [#4321]
+
 6.0.2
 ---------
 

lib/sidekiq/client.rb

@@ -99,8 +99,8 @@ module Sidekiq
 
       normed = normalize_item(items)
       payloads = items["args"].map.with_index { |args, index|
-        single_at = at.is_a?(Array) ? at[index] : at
-        copy = normed.merge("args" => args, "jid" => SecureRandom.hex(12), "at" => single_at, "enqueued_at" => Time.now.to_f)
+        copy = normed.merge("args" => args, "jid" => SecureRandom.hex(12), "enqueued_at" => Time.now.to_f)
+        copy["at"] = (at.is_a?(Array) ? at[index] : at) if at
 
         result = process_single(items["class"], copy)
         result || nil
@@ -193,7 +193,7 @@ module Sidekiq
     end
 
     def atomic_push(conn, payloads)
-      if payloads.first["at"]
+      if payloads.first.key?("at")
         conn.zadd("schedule", payloads.map { |hash|
           at = hash.delete("at").to_s
           [at, Sidekiq.dump_json(hash)]