add tests

lang/funcs/string_test.go

@@ -87,6 +87,12 @@ func TestTemplate(t *testing.T) {
 			cty.NilVal,
 			`vars map does not contain key "name", referenced at str:1,10-14`,
 		},
+		{
+			cty.StringVal("\xDF"),
+			cty.EmptyObjectVal,
+			cty.NilVal,
+			`str:1,1-2: Invalid character encoding; All input files must be UTF-8 encoded. Ensure that UTF-8 encoding is selected in your editor., and 1 other diagnostic(s)`,
+		},
 		{
 			cty.StringVal(""),
 			cty.MapVal(map[string]cty.Value{
@@ -95,6 +101,12 @@ func TestTemplate(t *testing.T) {
 			cty.StringVal(""),
 			``,
 		},
+		{
+			cty.NilVal,
+			cty.EmptyObjectVal,
+			cty.NilVal,
+			`argument must not be null`,
+		},
 		{
 			cty.StringVal("Hello, ${name}!"),
 			cty.MapVal(map[string]cty.Value{