core: Skip ignore_changes handling for create actions

It doesn't make sense to ignore_changes when the prior value is null,
since we have to create something before we can ignore changes to it.

This change is verified by TestContext2Apply_ignoreChangesWildcard.

terraform/context_apply_test.go

@@ -9002,15 +9002,14 @@ func TestContext2Apply_ignoreChangesWildcard(t *testing.T) {
 	})
 
 	if p, diags := ctx.Plan(); diags.HasErrors() {
-		t.Fatalf("diags: %s", diags.Err())
+		logDiagnostics(t, diags)
+		t.Fatal("plan failed")
 	} else {
 		t.Logf(legacyDiffComparisonString(p.Changes))
 	}
 
 	state, diags := ctx.Apply()
-	if diags.HasErrors() {
-		t.Fatalf("diags: %s", diags.Err())
-	}
+	assertNoErrors(t, diags)
 
 	mod := state.RootModule()
 	if len(mod.Resources) != 1 {

terraform/eval_diff.go

@@ -415,6 +415,12 @@ func (n *EvalDiff) Eval(ctx EvalContext) (interface{}, error) {
 }
 
 func (n *EvalDiff) processIgnoreChanges(schema *configschema.Block, prior, proposed cty.Value) (cty.Value, tfdiags.Diagnostics) {
+	// ignore_changes only applies when an object already exists, since we
+	// can't ignore changes to a thing we've not created yet.
+	if prior.IsNull() {
+		return proposed, nil
+	}
+
 	ignoreChanges := n.Config.Managed.IgnoreChanges
 	ignoreAll := n.Config.Managed.IgnoreAllChanges