fix(custom-property): return error for empty property_value instead of panicking

resource_github_repository_custom_property.go:72 unconditionally
dereferenced propertyValue[0] for scalar property_type values
(string, single_select, url, true_false). Terraform configurations
that supplied property_value as an empty list — or where a computed
value resolved to empty at apply time — crashed the provider with:

    panic: runtime error: index out of range [0] with length 0
    resourceGithubRepositoryCustomPropertyCreate ... +0x698

OpenTofu / Terraform recovers from this as a plugin crash, leaves
dangling resource state, and reports it as an internal provider
error rather than a user-facing configuration error.

Guard the index with an explicit length check that returns a clear
Terraform error naming the offending property_type. Multi_select is
unaffected — an empty slice there is intentionally allowed.

Refs integrations/terraform-provider-github issue 3358.

Author: SAY-5

github/resource_github_repository_custom_property.go

@@ -69,6 +69,9 @@ func resourceGithubRepositoryCustomPropertyCreate(d *schema.ResourceData, meta a
 	// The propertyValue can either be a list of strings or a string
 	switch propertyType {
 	case github.PropertyValueTypeString, github.PropertyValueTypeSingleSelect, github.PropertyValueTypeURL, github.PropertyValueTypeTrueFalse:
+		if len(propertyValue) == 0 {
+			return fmt.Errorf("property_value must contain at least one element for property_type %q", propertyType)
+		}
 		customProperty.Value = propertyValue[0]
 	case github.PropertyValueTypeMultiSelect:
 		customProperty.Value = propertyValue