[metric][fix]to_json for empty vector #1605

Job	Run time
ubuntu_clang (Release, OFF, ON)	3m 50s
ubuntu_clang_for_liburing (Release, OFF, ON)	4m 53s
ubuntu_clang10 (Release)	29s
ubuntu_clang_for_liburing (Debug, OFF, ON)	3m 39s
ubuntu_clang_pb (Release, OFF, ON)	1m 5s
ubuntu_clang10 (Debug)	42s
ubuntu_clang_pb (Release, OFF, OFF)	1m 1s
ubuntu_clang (Release, OFF, OFF)	3m 36s
ubuntu_clang_pb (Debug, OFF, ON)	1m 6s
ubuntu_clang (Debug, OFF, ON)	4m 32s
ubuntu_clang_pb (Debug, OFF, OFF)	1m 2s
ubuntu_clang (Debug, OFF, OFF)	3m 45s
	29m 40s

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