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

[notJoon]

Fixed to throw an error when trying to convert the type of nil, such as int(nil) or bool(nil) and so on.

related with #1341

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
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (789f4de) 47.91% compared to head (73b28ae) 47.90%.
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1347      +/-   ##
==========================================
- Coverage   47.91%   47.90%   -0.01%     
==========================================
  Files         372      368       -4     
  Lines       63019    62130     -889     
==========================================
- Hits        30194    29762     -432     
+ Misses      30363    29951     -412     
+ Partials     2462     2417      -45     

Files	Coverage Δ
gnovm/pkg/gnolang/values_conversions.go	3.83% <ø> (-0.83%)	⬇️

... and 38 files with indirect coverage changes

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[thehowl]

mostly LGTM, I would just change the place for the tests :)

[mvertes]

I think that this PR is made un-necessary if we apply first #1177 which fixes the root cause of attempt to convert untyped values, specially the block here

Note that converting undefined values is still legitimate in Go if you convert nil to a pointer, a slice or array type, or a map for example. Those cases should not be excluded as you did.

[thehowl]

Closing this PR.

With Marc's work on #1177 and Maxwell's work on #1426, we have already fixed the issues tackled by this PR.

@notJoon, you are still welcome to open another PR which only adds the test that you added for this one :)