feat(gnolang): print nil slices as undefined #2637

Job	Run time
build (1.20.x, GOARCH=amd64 GOOS=linux, gno)	28s
build (1.20.x, GOARCH=wasm GOOS=js, gno)	26s
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	21s
test (1.20.x, _test.cmd)	1m 13s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	26s
test (1.20.x, _test.pkg)	43s
test (1.20.x, _test.gnolang.native)	38s
test (1.20.x, _test.gnolang.stdlibs)	58s
test (1.20.x, _test.gnolang.realm)	45s
test (1.20.x, _test.gnolang.pkg0)	2m 5s
test (1.20.x, _test.gnolang.pkg1)	6m 27s
test (1.20.x, _test.gnolang.pkg2)	4m 16s
test (1.20.x, _test.gnolang.other)	35s
test (1.21.x, _test.cmd)	1m 6s
test (1.21.x, _test.pkg)	42s
test (1.21.x, _test.gnolang.native)	36s
test (1.21.x, _test.gnolang.stdlibs)	1m 0s
test (1.21.x, _test.gnolang.realm)	36s
test (1.21.x, _test.gnolang.pkg0)	1m 43s
test (1.21.x, _test.gnolang.pkg1)	7m 43s
test (1.21.x, _test.gnolang.pkg2)	4m 0s
test (1.21.x, _test.gnolang.other)	28s
upload-coverage	9s
	37m 24s

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