helper/schema: fix failing tests

helper/schema/schema.go

@@ -620,6 +620,9 @@ func (m schemaMap) diffString(
 	if o != nil && n == nil {
 		removed = true
 	}
+	if removed && schema.Computed {
+		return nil
+	}
 
 	diff.Attributes[k] = schema.finalizeDiff(&terraform.ResourceAttrDiff{
 		Old:        os,

helper/schema/schema_test.go

@@ -864,6 +864,43 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"config_vars": &Schema{
+					Type: TypeMap,
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"config_vars.foo": "bar",
+				},
+			},
+
+			Config: map[string]interface{}{
+				"config_vars": []interface{}{
+					map[string]interface{}{
+						"bar": "baz",
+					},
+				},
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"config_vars.foo": &terraform.ResourceAttrDiff{
+						Old:        "bar",
+						NewRemoved: true,
+					},
+					"config_vars.bar": &terraform.ResourceAttrDiff{
+						Old: "",
+						New: "baz",
+					},
+				},
+			},
+
+			Err: false,
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"config_vars": &Schema{