fix(gnovm): improve error message for nil assignment in variable declaration

[omarsy]

…aration

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[codecov]

Codecov Report

Attention: Patch coverage is 88.96552% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/types.go	52.63%	9 Missing ⚠️
gnovm/pkg/gnolang/preprocess.go	93.61%	6 Missing ⚠️
gnovm/pkg/gnolang/type_check.go	96.87%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[ltzmaxwell]

can you add some more tests like a + nil, etc. ? I guess there would be no more side effect but it's still worth to be clearer.

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[thehowl]

leaving @ltzmaxwell for final review/merge

[ltzmaxwell]

LGTM, thank you for fixing this.