Fix failing import acceptance test

While implementing #111, we noticed that the import acceptance test for
the `tfexec` package failed.
#111 (comment)

After investigation, I found that the time provider was significantly
rewritten in v0.9.0 to migrate to the new `terraform-plugin-framework`.
hashicorp/terraform-provider-time#112

As a result, the implementation of import for the `time_static` resource
now implicitly sets the `triggers` attribute to an empty map `{}`
instead of `null`.

I’m not sure if this change was intentional or not, though; we can
simply accept this change because there is no essential difference as
long as the import success with no change.

tfexec/terraform_import_test.go

@@ -159,7 +159,11 @@ func TestTerraformCLIImport(t *testing.T) {
 func TestAccTerraformCLIImport(t *testing.T) {
 	SkipUnlessAcceptanceTestEnabled(t)
 
-	source := `resource "time_static" "foo" {}`
+	source := `
+resource "time_static" "foo" {
+  triggers = {}
+}
+`
 	e := SetupTestAcc(t, source)
 	terraformCLI := NewTerraformCLI(e)