core: EvalConfigProvider shouldn't crash if its provider isn't set
core: EvalConfigProvider shouldn't crash if its provider isn't set

While there's no good reason for this to happen in practice, it can arise
in tests if mocks aren't set up quite right, and so we'll catch it and
report it nicely to make test debugging a little easier.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk