Fix nil pointers when time.Duration could be nil

[Cyb3r-Jak3]

Fixes #2487

There are some cases when a returned time.Duration could be nil for origin request config. I wasn't able to get the nil error 100% of the time but this should handle all cases.

[github-actions]

changelog detected ✅

[jacobbednarz]

cc @obezuk

[Cyb3r-Jak3]

This will need to be updated with cloudflare/cloudflare-go#1303

[jacobbednarz]

acceptance tests all passing

TF_ACC=1 go test ./internal/sdkv2provider -v -run "^TestAccCloudflareTunnelConfig_" -count 1 -timeout 120m -parallel 1
=== RUN   TestAccCloudflareTunnelConfig_Full
--- PASS: TestAccCloudflareTunnelConfig_Full (9.42s)
=== RUN   TestAccCloudflareTunnelConfig_Short
--- PASS: TestAccCloudflareTunnelConfig_Short (8.09s)
=== RUN   TestAccCloudflareTunnelConfig_NilPointer
--- PASS: TestAccCloudflareTunnelConfig_NilPointer (7.88s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/sdkv2provider	26.383s

[github-actions]

This functionality has been released in v4.9.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!