fix: Create private_ipv6_egress routes only when having at least one private subnet

[RunCor399]

Description

Private IPv6 Egress routes are now only created when having at least a private subnet

Motivation and Context

Version 5.7.1 of the module solved the issue #1058 by limiting the amount of IPv6 egress routes created to the amount of NAT gateways provisioned (hence to the amount of Route Tables created).

However the condition that regulates the number of IPv6 egress routes created doesn't check if the creation of any private subnet is actually wanted

• Resolves Error in function call for private_ipv6_egress in version 5.7.1 #1061

Breaking Changes

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
• I have executed pre-commit run -a on my pull request

[bryantbiggs]

how has this been tested and validated

[RunCor399]

how has this been tested and validated

I have run the same example quoted in issue #1061 before and after my modification.

[wiseelf]

Maybe it makes sense to set create_egress_only_igw to false by default?

[RunCor399]

Maybe it makes sense to set create_egress_only_igw to false by default?

I believe that since create_egress_only_igw is also used in the database_ipv6_egress route, setting it to false by default may cause problems when having a setup with database subnets and no private subnets (and viceversa). In the latter scenario create_egress_only_igw would have to be set to true, thus leading to the creation of routes for both private and db subnets

[antonbabenko]

This PR is included in version 5.7.2 🎉

[github-actions]

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