Allow users to set the provider-level `scopes` argument as an empty array and have that be used, instead of falling back to default scopes

[SarahFrench]

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Description

Currently the scopes provider-level argument can be set as an empty array. In this case the provider-configuration logic detects that the array has length 0 and then uses default scopes instead.

This might mean that users purposefully set no scopes in the provider block and believe that no scopes are granted (?) but in fact the default scopes are used.

Either we need to:

• Add validation on the value of scopes so that [] was no longer valid. This would avoid users being mislead about what's happening in the provider.
• Allow the empty array to be used in the logic, so that the client is configured with zero scopes included.

New or Affected Resource(s)

• google_XXXXX

Potential Terraform Configuration

This is currently valid

provider "google" {
    scopes = []
}

and effectively is the same as this

provider "google" {
    scopes = [
	"https://www.googleapis.com/auth/cloud-platform",
	"https://www.googleapis.com/auth/userinfo.email"
    ]
}

because those are the default scopes

References

GoogleCloudPlatform/magic-modules#11860 (comment)

[rileykarson]

I think we would want to accept nil/null (take default) as distinct from empty, empty is explicit empty, and other nonempty arrays are the same as before.

[SarahFrench]

Notes from discussion: We were unsure if the SDKv2 could distinguish between [] and null for lists, similar to how the SDK cannot differentiate between "" and null for strings. That wasn't a limitation discussed in the original PR (GoogleCloudPlatform/magic-modules#1215). If that's the case we should discuss ways to use the PF type system to help distinguish the empty array from a null (where defaults should be used instead).

If the SDKv2 can distinguish [] versus null arrays then the change will be pretty simple, though is still a breaking change as it'll result in altered behaviour.

Here is where the default scopes are used if the provided scopes array have length == 0

terraform-provider-google/google/transport/config.go

Lines 1354 to 1356 in 9abc47a

	if len(c.Scopes) == 0 {
	c.Scopes = DefaultClientScopes
	}