feat(gnolang): print nil slices as undefined #1706

Job	Run time
test (1.20.x, _test.flappy)	33s
build (1.20.x, amd64, linux, ./pkg/amino/cmd/aminoscan)	7s
test (1.20.x, _test.pkg.amino)	1m 36s
build (1.20.x, amd64, linux, ./pkg/amino/cmd/goscan)	12s
test (1.20.x, _test.pkg.bft)	2m 36s
build (1.20.x, amd64, linux, ./pkg/autofile/cmd)	12s
test (1.20.x, _test.pkg.others)	2m 41s
build (1.20.x, amd64, linux, ./pkg/iavl/cmd/iaviewer)	16s
test (1.21.x, _test.flappy)	28s
build (1.21.x, amd64, linux, ./pkg/amino/cmd/aminoscan)	17s
test (1.21.x, _test.pkg.amino)	1m 30s
build (1.21.x, amd64, linux, ./pkg/amino/cmd/goscan)	5s
test (1.21.x, _test.pkg.bft)	2m 41s
build (1.21.x, amd64, linux, ./pkg/autofile/cmd)	13s
build (1.21.x, amd64, linux, ./pkg/iavl/cmd/iaviewer)	17s
test (1.21.x, _test.pkg.others)	2m 39s
upload-coverage	4s
	16m 27s

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