r/aws_wafv2_web_acl(test): cases for rule_json expander

```console
% go test -count=1 -v ./internal/service/wafv2/... -run="Test_expandWebACLRulesJSON"

--- PASS: Test_expandWebACLRulesJSON (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/empty_string (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/single_null_object (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/empty_array (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/single_empty_object (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/valid_and_empty_object (0.00s)
    --- PASS: Test_expandWebACLRulesJSON/valid_object (0.00s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/wafv2      5.969s
```

internal/service/wafv2/flex_test.go

@@ -0,0 +1,98 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package wafv2
+
+import (
+	"testing"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	awstypes "github.com/aws/aws-sdk-go-v2/service/wafv2/types"
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+)
+
+func Test_expandWebACLRulesJSON(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		rawRules string
+		want     []awstypes.Rule
+		wantErr  bool
+	}{
+		"empty string": {
+			rawRules: "",
+			wantErr:  true,
+		},
+		"empty array": {
+			rawRules: "[]",
+			want:     []awstypes.Rule{},
+		},
+		"single empty object": {
+			rawRules: "[{}]",
+			wantErr:  true,
+		},
+		"single null object": {
+			rawRules: "[null]",
+			wantErr:  true,
+		},
+		"valid object": {
+			rawRules: `[{"Action":{"Count":{}},"Name":"rule-1","Priority":1,"Statement":{"RateBasedStatement":{"AggregateKeyType":"IP","EvaluationWindowSec":600,"Limit":10000,"ScopeDownStatement":{"GeoMatchStatement":{"CountryCodes":["US","NL"]}}}},"VisibilityConfig":{"CloudwatchMetricsEnabled":false,"MetricName":"friendly-rule-metric-name","SampledRequestsEnabled":false}}]`,
+			want: []awstypes.Rule{
+				{
+					Name:     aws.String("rule-1"),
+					Priority: 1,
+					Action: &awstypes.RuleAction{
+						Count: &awstypes.CountAction{},
+					},
+					Statement: &awstypes.Statement{
+						RateBasedStatement: &awstypes.RateBasedStatement{
+							Limit:               aws.Int64(10000),
+							AggregateKeyType:    awstypes.RateBasedStatementAggregateKeyType("IP"),
+							EvaluationWindowSec: 600,
+							ScopeDownStatement: &awstypes.Statement{
+								GeoMatchStatement: &awstypes.GeoMatchStatement{
+									CountryCodes: []awstypes.CountryCode{"US", "NL"},
+								},
+							},
+						},
+					},
+					VisibilityConfig: &awstypes.VisibilityConfig{
+						CloudWatchMetricsEnabled: false,
+						MetricName:               aws.String("friendly-rule-metric-name"),
+						SampledRequestsEnabled:   false,
+					},
+				},
+			},
+		},
+		"valid and empty object": {
+			rawRules: `[{"Action":{"Count":{}},"Name":"rule-1","Priority":1,"Statement":{"RateBasedStatement":{"AggregateKeyType":"IP","EvaluationWindowSec":600,"Limit":10000,"ScopeDownStatement":{"GeoMatchStatement":{"CountryCodes":["US","NL"]}}}},"VisibilityConfig":{"CloudwatchMetricsEnabled":false,"MetricName":"friendly-rule-metric-name","SampledRequestsEnabled":false}},{}]`,
+			wantErr:  true,
+		},
+	}
+
+	ignoreExportedOpts := cmpopts.IgnoreUnexported(
+		awstypes.Rule{},
+		awstypes.RuleAction{},
+		awstypes.CountAction{},
+		awstypes.Statement{},
+		awstypes.RateBasedStatement{},
+		awstypes.GeoMatchStatement{},
+		awstypes.VisibilityConfig{},
+	)
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := expandWebACLRulesJSON(tc.rawRules)
+			if (err != nil) != tc.wantErr {
+				t.Errorf("expandWebACLRulesJSON() error = %v, wantErr %v", err, tc.wantErr)
+				return
+			}
+			if diff := cmp.Diff(got, tc.want, ignoreExportedOpts); diff != "" {
+				t.Errorf("unexpected diff (+wanted, -got): %s", diff)
+			}
+		})
+	}
+}