helper/schema: fix failing tests

/cc @svanharmelen - I think some logic changed after my refactor. I now
return Exists: true when Computed: true but the value might be blank to
note that the FieldReader FOUND a value, its just unknown. I think
before it didn't do that so the logic for GetOk has to be "does it exist
and is it _not_ computed"

Seems weird because I just realized there is no way to get the OLD value
of something if it is being computed now, but I looked and there are
tests that verify this and they're like... test #5 of Get. So, they're
not new meaning that must've been expected behavior? Hm. Let me know if
you find any other issues from acceptance tests

helper/schema/resource_data.go

@@ -91,7 +91,7 @@ func (d *ResourceData) GetChange(key string) (interface{}, interface{}) {
 // The second result should be checked to determine this information.
 func (d *ResourceData) GetOk(key string) (interface{}, bool) {
 	r := d.getRaw(key, getSourceSet)
-	return r.Value, r.Exists
+	return r.Value, r.Exists && !r.Computed
 }
 
 func (d *ResourceData) getRaw(key string, level getSource) getResult {

helper/schema/schema_test.go

@@ -868,7 +868,7 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Diff: &terraform.InstanceDiff{
 				Attributes: map[string]*terraform.ResourceAttrDiff{
 					"ports.#": &terraform.ResourceAttrDiff{
-						Old:         "0",
+						Old:         "",
 						New:         "",
 						NewComputed: true,
 					},
@@ -1560,7 +1560,6 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Diff: &terraform.InstanceDiff{
 				Attributes: map[string]*terraform.ResourceAttrDiff{
 					"instances.#": &terraform.ResourceAttrDiff{
-						Old:         "0",
 						NewComputed: true,
 					},
 				},
@@ -1688,7 +1687,6 @@ func TestSchemaMap_Diff(t *testing.T) {
 						New: "1",
 					},
 					"route.~1.gateway.#": &terraform.ResourceAttrDiff{
-						Old:         "0",
 						NewComputed: true,
 					},
 				},