don't append nil elements to a diagnostic cty.Path

If a set element is nil in validateConfigNulls, we don't want to
append that element to the diagnostic path, since it doesn't offer any
useful info to the user.

helper/plugin/grpc_provider.go

@@ -1374,11 +1374,19 @@ func validateConfigNulls(v cty.Value, path cty.Path) []*proto.Diagnostic {
 		for it.Next() {
 			kv, ev := it.Element()
 			if ev.IsNull() {
+				// if this is a set, the kv is also going to be null which
+				// isn't a valid path element, so we can't append it to the
+				// diagnostic.
+				p := path
+				if !kv.IsNull() {
+					p = append(p, cty.IndexStep{Key: kv})
+				}
+
 				diags = append(diags, &proto.Diagnostic{
 					Severity:  proto.Diagnostic_ERROR,
 					Summary:   "Null value found in list",
 					Detail:    "Null values are not allowed for this attribute value.",
-					Attribute: convert.PathToAttributePath(append(path, cty.IndexStep{Key: kv})),
+					Attribute: convert.PathToAttributePath(p),
 				})
 				continue
 			}

helper/plugin/grpc_provider_test.go

@@ -1352,6 +1352,17 @@ func TestValidateNulls(t *testing.T) {
 			}),
 			Err: true,
 		},
+		{
+			Cfg: cty.ObjectVal(map[string]cty.Value{
+				"object": cty.ObjectVal(map[string]cty.Value{
+					"list": cty.SetVal([]cty.Value{
+						cty.StringVal("string"),
+						cty.NullVal(cty.String),
+					}),
+				}),
+			}),
+			Err: true,
+		},
 	} {
 		t.Run(strconv.Itoa(i), func(t *testing.T) {
 			d := validateConfigNulls(tc.Cfg, nil)