wafv2.WebAcl: tweak Set identity to suppress resource cycling

[t0yv0]

Fixes #3880

Deep investigation in pulumi/pulumi-terraform-bridge#1917 - the root cause bottoms out at inconsistent set element identity computed by the bridge as well as Terraform proper; there is some slight discrepancy in how the bridged providers work compared to pure TF that makes the result poor in Pulumi, however even under TF there are two different values computed for what the user perceives as the same data. This is a surgical change to that makes the set identity indifferent to whether block.custom_response is encoded as missing or an empty list, which works around the problem.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

Maintainer note: consult the runbook for dealing with any breaking changes.

[github-actions]

Is README.md missing any configuration options?

assumeRoleWithWebIdentity not found in Configuration section
customCaBundle not found in Configuration section
defaultTags not found in Configuration section
ec2MetadataServiceEndpoint not found in Configuration section
ec2MetadataServiceEndpointMode not found in Configuration section
httpProxy not found in Configuration section
httpsProxy not found in Configuration section
ignoreTags not found in Configuration section
noProxy not found in Configuration section
retryMode not found in Configuration section
s3UsEast1RegionalEndpoint not found in Configuration section
sharedConfigFiles not found in Configuration section
skipRequestingAccountId not found in Configuration section
stsRegion not found in Configuration section
tokenBucketRateLimiterCapacity not found in Configuration section
useDualstackEndpoint not found in Configuration section

Please add a description for each of these options to README.md. Details about them can be found in either the upstream docs or schema.json.

[t0yv0]

Closing in favor of pulumi/pulumi-terraform-bridge#1927

[t0yv0]

This was actually fundamentally incorrect per recent conversation - @corymhall - excluding elements from Set function will make changes to these elements always result in no-diff plans which is ultimately incorrect.