fix: Fix error for the null ordered_cache_behavior variable, make the default_cache_behavior variable nullable

[flora-five]

Description

• Make the default_cache_behavior variable nullable, to allow the creation of standalone resources like functions, policies
• Include the fix for the concat error for cache behaviors local variable fix: Avoid adding a null value to concat() function for cache behaviors local variable #179
• Extend the example with creation of standalone functions

Motivation and Context

Fixes #178 and #180
Supercedes #179 is accepted

Breaking Changes

No

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]

this is not a valid set of changes per https://github.com/hashicorp/terraform-provider-aws/blob/8927020aa6b462f30ecb8caed61308f2aa3f59d6/internal/service/cloudfront/distribution.go#L105-L107