Fix bug in DataLoader::Source#load_all when sending only falsy values #2602

Job	Run time
javascript_test	28s
postgres_test (gemfiles/rails_master.gemfile, 3.3, 1)	1m 46s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 58s
postgres_test (gemfiles/rails_7.1_postgresql.gemfile, 3.3)	1m 35s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 57s
lint	18s
system_tests	1m 47s
test (Gemfile, 3.2)	1m 35s
test (Gemfile, 2.7)	1m 39s
test (gemfiles/rails_7.0.gemfile, 3.1)	2m 11s
test (gemfiles/rails_master.gemfile, 3.2)	2m 33s
test (gemfiles/rails_7.0.gemfile, 3.2)	1m 57s
test (gemfiles/rails_7.1.gemfile, 3.3)	2m 20s
test (gemfiles/rails_master.gemfile, 3.3, 1)	2m 22s
test (gemfiles/rails_master.gemfile, 3.3, 1, 1)	2m 22s
	26m 48s

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