fix(gnovm): do not allow nil as type declaration #7393

Job	Run time
setup	7s
Run Main (github-bot) / Go Build / generated	6s
Run Main (github-bot) / Go Linter / lint	27s
Run Main (github-bot) / Go Test / test	25s
Run Main (gnodev) / Go Build / generated	11s
Run Main (gnodev) / Go Linter / lint	51s
Run Main (gnodev) / Go Test / test	1m 18s
Run Main (gnofaucet) / Go Build / generated	9s
Run Main (gnofaucet) / Go Linter / lint	38s
Run Main (gnofaucet) / Go Test / test	37s
Run Main (gnogenesis) / Go Build / generated	9s
Run Main (gnogenesis) / Go Linter / lint	35s
Run Main (gnogenesis) / Go Test / test	39s
Run Main (gnohealth) / Go Build / generated	10s
Run Main (gnohealth) / Go Linter / lint	26s
Run Main (gnohealth) / Go Test / test	30s
Run Main (gnokeykc) / Go Build / generated	5s
Run Main (gnokeykc) / Go Linter / lint	36s
Run Main (gnokeykc) / Go Test / test	39s
Run Main (gnomd) / Go Build / generated	10s
Run Main (gnomd) / Go Linter / lint	25s
Run Main (gnomd) / Go Test / test	26s
Run Main (gnomigrate) / Go Build / generated	5s
Run Main (gnomigrate) / Go Linter / lint	43s
Run Main (gnomigrate) / Go Test / test	40s
	11m 7s

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