Fix bug in DataLoader::Source#load_all when sending only falsy values #2601

Job	Run time
postgres_test (gemfiles/rails_master.gemfile, 3.3, 1)	2m 4s
lint	38s
postgres_test (gemfiles/rails_7.1_postgresql.gemfile, 3.3)	2m 4s
javascript_test	27s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 59s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 56s
system_tests	2m 26s
test (Gemfile, 3.2)	1m 53s
test (Gemfile, 2.7)	1m 41s
test (gemfiles/rails_7.0.gemfile, 3.1)	2m 10s
test (gemfiles/rails_master.gemfile, 3.2)	2m 29s
test (gemfiles/rails_7.0.gemfile, 3.2)	2m 22s
test (gemfiles/rails_7.1.gemfile, 3.3)	2m 13s
test (gemfiles/rails_master.gemfile, 3.3, 1)	2m 27s
test (gemfiles/rails_master.gemfile, 3.3, 1, 1)	2m 25s
	29m 14s

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