"IPv6 communication over VPC peering" PR broke module functionality without IPv6 specified

[adziura-ledger]

Describe the Bug

When you don't have IPv6 cidr specified for VPC module will not work because of this change
data.aws_vpc.requester[*].ipv6_cidr_block returns list with one empty element, and length will be always > 0
same for accepter
Changes introduced in PR #101

Expected Behavior

No changes if IPv6 CIDRs not defined.

Steps to Reproduce

Apply module (starting from version 0.20.1) for VPCs without IPv6 CIDRs defined.

Screenshots

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Environment

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Additional Context

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[NathanBaulch]

This also broke my ipv6-less VPC deployment with:

│ Error: "" is not a valid CIDR block: invalid CIDR address:
│
│   with module.peering.aws_route.accepter_ipv6[0],
│   on .terraform\modules\peering\accepter.tf line 128, in resource "aws_route" "accepter_ipv6":
│  128:   destination_ipv6_cidr_block = local.requester_ipv6_cidr_block_associations[count.index % local.requester_ipv6_cidr_block_associations_count]["cidr_block"]

Downgrading to v0.20.0 for now.