fix(gnolang): reject invalid undefined/nil type conversions #1572

Job	Run time
build (1.20.x, amd64, linux, ./pkg/amino/cmd/aminoscan)	7s
test (1.20.x, _test.flappy)	25s
build (1.20.x, amd64, linux, ./pkg/amino/cmd/goscan)	8s
test (1.20.x, _test.pkg.amino)	1m 33s
test (1.20.x, _test.pkg.bft)	21m 13s
build (1.20.x, amd64, linux, ./pkg/autofile/cmd)	7s
test (1.20.x, _test.pkg.others)	2m 35s
build (1.20.x, amd64, linux, ./pkg/iavl/cmd/iaviewer)	9s
test (1.21.x, _test.flappy)	30s
build (1.21.x, amd64, linux, ./pkg/amino/cmd/aminoscan)	12s
test (1.21.x, _test.pkg.amino)	1m 33s
build (1.21.x, amd64, linux, ./pkg/amino/cmd/goscan)	13s
test (1.21.x, _test.pkg.bft)	2m 39s
build (1.21.x, amd64, linux, ./pkg/autofile/cmd)	16s
test (1.21.x, _test.pkg.others)	2m 32s
build (1.21.x, amd64, linux, ./pkg/iavl/cmd/iaviewer)	13s
upload-coverage	0s
	34m 25s

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