Bug: Some logging providers were sending an empty placement to the Fastly API

[Integralist]

Problem:
When creating a logging provider the placement attribute is optional and the API will expect it to not be sent with an empty value, but that is what was happening in the latest Terraform v3 release.

Context :
The Terraform provider wraps all values in a pointer (as that is what go-fastly v7+ expects), but the zero value for an attribute (e.g. "" for the string type) is what will be wrapped inside the pointer and consequently sent to the API via go-fastly. Previously we had used the integration/acceptance tests to identify which attributes would cause API errors but what happened in this instance was that the tests were setting a value for that field and so no error was reported.

Solution:
Do as we have already done with the other logging providers and avoid setting the placement field on the go-fastly struct.

[Integralist]

I've run the integration test suite and it is passing.