Don't raise CrossSlotError when no keys given

When no keys are passed to a Redis command that accepts multiple keys,
that's an error. But it's not a cross-slot violation, as there are no
slots. By raising this error we were masking the true
Redis::CommandError underneath, which is bad for two reasons:

1. It's confusing to developers.
2. It leads to a different failure in production to development and
   test.

To elaborate a bit more on the second point: the purpose of the
CrossSlotError is to only be an _additional_ error in development and
test, but it shouldn't _replace_ underlying errors as that can cause
issues like this one.

lib/gitlab/instrumentation/redis_cluster_validator.rb

@@ -61,7 +61,7 @@ module Gitlab
             key_slot(args.first)
           end
 
-          unless key_slots.uniq.length == 1
+          if key_slots.uniq.many? # rubocop: disable CodeReuse/ActiveRecord
             raise CrossSlotError.new("Redis command #{command_name} arguments hash to different slots. See https://docs.gitlab.com/ee/development/redis.html#multi-key-commands")
           end
         end

spec/lib/gitlab/instrumentation/redis_cluster_validator_spec.rb

@@ -53,6 +53,7 @@ RSpec.describe Gitlab::Instrumentation::RedisClusterValidator do
       :del | [%w(foo bar)] | true # Arguments can be a nested array
       :del | %w(foo foo) | false
       :hset | %w(foo bar) | false # Not a multi-key command
+      :mget | [] | false # This is invalid, but not because it's a cross-slot command
     end
 
     with_them do