Caller: use lonely operator to safely call `#first` #161

Job	Run time
Check documentation syntax	8s
JRuby 9.4	1m 4s
Check the oldest supported RuboCop version	46s
Prism	34s
Ruby 2.7	41s
Ruby 3.0	29s
Ruby 3.1	30s
Ruby 3.2	32s
Ruby 3.3	28s
Ruby head	26s
	5m 38s

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