Fix Substr function call with length argument of zero to return ""

[nick96]

This issue was raised in terraform (hashicorp/terraform#24318) and it looks like it's a bug in stdlib.SubstrFunc's impl where the substr function returns the whole string then the length argument is 0.

I've got a fix which is hopefully useful, I thought that it'd be best to have the length check as high up as it can but there may be other factors I'm not considering (I've run the test suite and it's passing).

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   70.11%   70.21%   +0.09%     
==========================================
  Files          78       78              
  Lines        7078     7080       +2     
==========================================
+ Hits         4963     4971       +8     
+ Misses       1672     1666       -6     
  Partials      443      443

Impacted Files	Coverage Δ
cty/function/stdlib/string.go	41.05% <100%> (+0.62%)	⬆️
cty/convert/compare_types.go	100% <0%> (+7.14%)	⬆️

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[apparentlymart]

Thanks for looking into this, @nick96!

This fix looks good to me, and I'm going to merge it now. I suppose for the downstream Terraform issue the next step here would be to open a PR including an upgraded version of cty containing this fix. I'm intending to hold for a little longer before cutting the next cty release in case there are other changes to include in it, because the previous release was only last week, but one of the other changes awaiting release is also partially motivated by work in Terraform so we can resolve them both at once with a single PR to Terraform once that release is tagged.

I expect that from Terraform's perspective this fix could be considered as a breaking change to its configuration language (in case someone is already relying on zero being interpreted as "the remainder of the string").

[nick96]

Awesome! I'll raise the next steps issue on the Terraform issue,