Fix for empty keys on Redis#del

Prevents attempts to call Redis `del` with no arguments.

lib/gitlab/set_cache.rb

@@ -14,7 +14,10 @@ module Gitlab
       "#{key}:set"
     end
 
+    # Returns the number of keys deleted by Redis
     def expire(*keys)
+      return 0 if keys.empty?
+
       with do |redis|
         keys = keys.map { |key| cache_key(key) }
         unlink_or_delete(redis, keys)

spec/lib/gitlab/repository_set_cache_spec.rb

@@ -88,6 +88,12 @@ describe Gitlab::RepositorySetCache, :clean_gitlab_redis_cache do
       end
     end
 
+    context 'no keys' do
+      let(:keys) { [] }
+
+      it { is_expected.to eq(0) }
+    end
+
     context "unlink isn't supported" do
       before do
         allow_any_instance_of(Redis).to receive(:unlink) { raise ::Redis::CommandError }