Fix conversion issue flattenAddons func in client resource

[sergiught]

Description

Fixes #139

Fixes issue with the logout property on flattenAddons where we're trying to typecast it always to a map[string]interface{} without checking if it was actually present in the payload.

Checklist

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Auth0 Code of Conduct

• I have read and agreed to the terms within the Auth0 Code of Conduct

Auth0 General Contribution Guidelines

• I have read the Auth0 General Contribution Guidelines

Changes include test coverage?

• Yes
• Not needed

Does the description provide the correct amount of context?

• Yes, the description provides enough context for the reviewer to understand what these changes accomplish

Have you updated the documentation?

• Yes, I've updated the appropriate docs
• Not needed

Is this code ready for production?

• Yes, all code changes are intentional and no debugging calls are left over

[willvedd]

IMO, I think it would've made sense to keep this as the default testAccClientConfig and then label the above testAccClientConfigWithoutAddonsWithoutSAMLPLogout but just a small amount of 🚲 🏠 'ing.