Fix CoalesceList panic when passed null arguments

[alisdair]

The coalescelist function was moved here from hashicorp/terraform in #37. While the Terraform function documentation doesn't specify whether or not null arguments are permitted, the implementation does allow them:

go-cty/cty/function/stdlib/collection.go

Lines 205 to 213 in 4a53886

	var CoalesceListFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
	Name: "vals",
	Type: cty.DynamicPseudoType,
	AllowUnknown: true,
	AllowDynamicType: true,
	AllowNull: true,
	},

However as reported in hashicorp/terraform#26988, passing a null argument causes a panic. This commit fixes the crash and adds test coverage for the function.

[codecov]

Codecov Report

Merging #81 (c4f4382) into main (4a53886) will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   70.36%   70.69%   +0.32%     
==========================================
  Files          79       79              
  Lines        6550     6552       +2     
==========================================
+ Hits         4609     4632      +23     
+ Misses       1497     1476      -21     
  Partials      444      444              

Impacted Files	Coverage Δ
cty/function/stdlib/collection.go	26.02% <100.00%> (+3.86%)	⬆️

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[apparentlymart]

Thanks for this!

These functions that originated in Terraform do unfortunately seem to have inherited some quirks from their origins as HIL functions 😖 but I agree that skipping over the nulls as if they were empty lists seems like a reasonable behavior given the mission of this function, and is consistent with how coalesce handles null and empty strings.