allow evaluation of 0 instances during apply

While this was easier to spot during plan, it is also possible to
evaluate resources with 0 instances during apply as well.

This doesn't effect the failure when scaling CBD instances, it only
changes the fact that the inconsistent value is no longer unknown.

terraform/context_apply_test.go

@@ -11912,3 +11912,66 @@ resource "test_instance" "b" {
 		t.Fatalf("apply errors: %s", diags.Err())
 	}
 }
+
+func TestContext2Apply_removeReferencedResource(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+variable "ct" {
+}
+
+resource "test_resource" "to_remove" {
+  count = var.ct
+}
+
+resource "test_resource" "c" {
+  value = join("", test_resource.to_remove[*].id)
+}
+`})
+
+	p := testProvider("test")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+		Variables: InputValues{
+			"ct": &InputValue{
+				Value: cty.NumberIntVal(1),
+			},
+		},
+	})
+
+	if _, diags := ctx.Plan(); diags.HasErrors() {
+		t.Fatalf("plan errors: %s", diags.Err())
+	}
+
+	state, diags := ctx.Apply()
+	if diags.HasErrors() {
+		t.Fatalf("apply errors: %s", diags.Err())
+	}
+
+	ctx = testContext2(t, &ContextOpts{
+		Config: m,
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+		Variables: InputValues{
+			"ct": &InputValue{
+				Value: cty.NumberIntVal(0),
+			},
+		},
+		State: state,
+	})
+
+	if _, diags := ctx.Plan(); diags.HasErrors() {
+		t.Fatalf("plan errors: %s", diags.Err())
+	}
+
+	_, diags = ctx.Apply()
+	if diags.HasErrors() {
+		t.Fatalf("apply errors: %s", diags.Err())
+	}
+}

terraform/evaluate.go

@@ -648,12 +648,10 @@ func (d *evaluationStateData) GetResource(addr addrs.Resource, rng tfdiags.Sourc
 
 	if rs == nil {
 		switch d.Operation {
-		case walkPlan:
+		case walkPlan, walkApply:
-			// During plan as we evaluate each removed instance they are removed
+			// During plan and apply as we evaluate each removed instance they
-			// from the temporary working state. Since we know there there are
+			// are removed from the working state. Since we know there are no
-			// no instances, and resources might be referenced in a context
+			// instances, return an empty container of the expected type.
-			// that needs to be known during plan, return an empty container of
-			// the expected type.
 			switch {
 			case config.Count != nil:
 				return cty.EmptyTupleVal, diags