core: fix NodePlannableResourceInstance data reads
core: fix NodePlannableResourceInstance data reads

The state after EvalReadDataDiff is no longer nil during plan, which
means that we can't use that as a proxy for requiring the diff.

Rather than exiting early to save the EvalWriteState and EvalWriteDiff
evaluations, continue normally regardless to ensure we have the latest
diff and state after the plan. This also aligns the data data source
handling with that of the managed resource.

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