Use `"%empty"?` instead of `empty` #1407

Job	Run time
test-memory (head)	47s
check-misc (head)	7s
test-ruby (head, master)	6m 24s
steep-check (head)	11s
test-ruby (3.0, master)	6m 4s
test-windows (head)	1m 38s
ruby-versions / Generate Ruby versions	2s
test-cpp (head, g++)	12s
test-cpp (head, clang++)	18s
test (2.5)	24s
test (2.6)	27s
test (2.7)	25s
test (3.0)	17s
test (3.1)	17s
test (3.2)	18s
test (3.3)	12s
test (head)	19s
	18m 22s

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