fix(gnolang): allow comparisons using uninitialized string values

[thehowl]

The provided additional test, without the patch, fails with the following error:

=== RUN   TestFiles/comp3.gno
Machine.RunMain() panic: interface conversion: gnolang.Value is nil, not gnolang.StringValue
[...]
goroutine 180 [running]:
runtime/debug.Stack()
        /usr/lib/go/src/runtime/debug/stack.go:24 +0x65
[...]
panic({0xb91680, 0xc001fa7bf0})
        /usr/lib/go/src/runtime/panic.go:884 +0x213
github.com/gnolang/gno/gnovm/pkg/gnolang.isLss(0xc0054b4050, 0xc0054b4078)
        /home/howl/oc/gno2/gnovm/pkg/gnolang/op_binary.go:492 +0x431

This seems to be because internally, the string value is "uninitialized" (hopefully right word here). This is in opposition to an initialised string, as would happen for the statement x := "".

This PR changes the behaviour for comparisons inside of op_binary (<, >, <=, >=) to use GetString instead of a type assertion of TypedValue.V to a StringValue. This is in line with what is already done inside of isEql, introduced in this commit:

da6520f#diff-7cc5c6bc5496b6ad9ed55e04e1cdf2f0d1d5954af21be5bc38ef3c46389149a9L358

git blame points instead this part of code inside of the comparisons functions to go back to the initial commit.

Additionally, I looked for other cases where we are currently doing type assertions directly into a StringValue, and there was one in the implementation of append. Since this is a special case and requires having a native value as the first argument, I haven't written a test for it, but the change should be safe as GetString() internally just does the type assertion, but checks for tv.V == nil first.

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

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ce258b1) 47.31% compared to head (f52130f) 47.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1132      +/-   ##
==========================================
+ Coverage   47.31%   47.33%   +0.01%     
==========================================
  Files         367      367              
  Lines       62118    62118              
==========================================
+ Hits        29394    29402       +8     
  Misses      30325    30325              
+ Partials     2399     2391       -8     

Files	Coverage Δ
gnovm/pkg/gnolang/uverse.go	19.58% <0.00%> (ø)
gnovm/pkg/gnolang/op_binary.go	4.79% <0.00%> (ø)

... and 7 files with indirect coverage changes

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[thehowl]

Codecov will mark this as not tested without #1122 . I'd say this is still good to merge, though. (Moved to review meeting col)