MINOR: add omitempty to epoll_mask_events #909

Job	Run time
generate-specification	4s
generate-models	1m 27s
HAProxy check commit message	0s
go mod tidy	0s
license-check	0s
lint-spec	0s
golangci-lint	0s
Test equal	0s
Tests	0s
Build	0s
e2e on HAProxy	0s
	1m 31s

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